This summer seems to be offering highly anticipated blockbuster events for every taste. If you’re a comic-book fan, there’s the new Batman movie. If you’ve been waiting since 1991 for new music from Guns N’ Roses, we learn that a track from a long-delayed album will be on the “Rock Band 2” video game. And if you’re a gadget geek, you were standing in line on July 15 for first crack at the new Apple iPhone 3G, cheaper, faster and maybe more Web-enabled than its original touch-screen counterpart.

But fans of the iPhone, and other smartphones able to access the Web, are growing less geeky and more mainstream all the time, and marketers who are waiting for the mobile Internet to go big should stop waiting. Those are the key takeaways of a new report compiled by Nielsen Mobile from diverse research sources within the company, giving new hope to marketers who would like to start pushing content and brand messaging to users who aren’t at their laptop or home PC.

The report, “Critical Mass: The Worldwide State of the Mobile Web”, takes a look at May 2008 mobile Web usage in the U.S. and in 15 other countries in Europe, Asia, South America, with New Zealand in the mix too.

Since we’ve long heard how far behind the mobile curve Americans are compared to some Asian counterparts, it comes as a surprise that the Nielsen figures put the U.S. at the top of the mobile Web heap. (That heap did not include either Japan or South Korea.)

Nevertheless, the U.S. leads a high-profile league of nations, with 15.6% of its 254 million wireless phone subscribers getting to the Internet via their handsets in the month. The United Kingdom was the second most penetrated market, with 12.9% accessing the mobile Web, followed by Italy (11.9%), Russia (11.2%), Spain (10.8%), Thailand (10%), France (9.6%) and China (6.8%).

“We believe mobile Internet has reached a critical mass as an advertising medium in the U.S.,” the report says. “As of May 2008, there were 40 million active users of the mobile Internet in the U.S. with individual sites that attract millions of unique users. This provides scalable marketing potential with demographic breadth.”

In other words, you could at least make an argument that mobile marketing’s time has come—even if the campaigns haven’t.

“There’s a weird hiccup in the marketplace right now, in that the audiences and the capabilities have arrived,” says Nic Covey, director of insights for Nielsen Mobile and the primary author of the report. “But I get the sense that some avails are not being activated, and some CMOs are slow to put their dollars where their mouths are.”

The report looks both at the current state of mobile Web usage and at some factors that should drive its wider adoption as time ensues. And it picks up on some interesting points.

For example those 40 million Americans using the mobile Internet in May were a subset of the 95 million who paid for mobile Web access during Q1 2008, either on a per-use basis or as part of their monthly subscription price. That’s up more than one-fourth from the 75 million who were paying for mobile Web use in the same period last year.

In other words, a relatively high proportion of U.S. subscribers are paying for a mobile service that they’re not using. While that may be a short-term boon for wireless carriers, it could also lead to lots of churned accounts as users switch providers for price.

To avoid that turnover—something they’ve already been through on the voice front– carriers are starting to offer flat-fee data plans that place either no limit or a very high one on the minutes users can spend doing anything other than talking on their phones. Sprint Nextel leads that pack with a $100 plan for unlimited voice, text and data.

If other carriers move from a la carte to a buffet business model, that could have benefits for mobile marketers too by opening up the channel to more users.

“Americans love all-you-can-eat,” Covey says. “The model of charging by the kilobit has worked well in other countries, but our market here will surely move toward more unlimited packages, because that’s what consumers will demand.”

Getting people to user the mobile Internet is only part of the problem, of course. Another element for marketers will be figuring out where they go on the Web and how that behavior differs from home-bound users.

For one thing, mobile Web services offer users a choice: either stay within the confines of the carrier’s preselected menu of Web sites (called “decks”, as in staying “on deck”) or go out and reach the Web directly by inputting URLs or searching via Google or Yahoo! Those last have not been easy to do because they require thumb-typing names or Web addresses—and in fact triple-thumbing on a standard phone keyboard. But as users opt for devices with full (if small) QWERTY boards, climbing off the deck is getting easier to do.

“We believe the carriers face a real challenge in the coming year to keep interested consumers on their decks,” Covey says. “Many mobile users will find their way off-deck and into the open Web as quickly as they can. But other consumers will be thankful that someone has put information into a hierarchy for them, so they can just hit ‘News/national/politics’ and be where they want to be.”

But that number who stay in the carriers’ walled gardens may be smaller than once thought. Nielsen reports that only 17% of users say they restrict their mobile Web use to the carrier portals. That compares to 40% who say they find mobile sites through mobile search engines. And 22% says they get to sites over their mobile phones by simply typing in a URL they already know from their PC-based use—a fact that may lead brands to start getting serious about optimizing their Web sites for mobile access.

Where will they want to go on their handsets? If the current state of affairs persists, mostly to brand portals such as Yahoo! and Google. Those portals had 36 million unique U.S. visitors in May of this year (89% audience share). Web e-mail was the next most popular category for the month, garnering 26 million U.S. uniques.

Other mobile Web categories with high traffic for the month included weather (16.8 million unique visitors), news and politics sites (13 million), search engines (11.8 million) and city guides or maps (9.9 million).

But as mobile Internet use grows beyond the early adopter stage, Covey says, other categories may come up in popularity. In particular, social networking mobile sites and mobile banking sites may be poised for substantial growth. Right now each category attracts about 5 million unique visitors monthly.

Looking beyond categories, the actual sites visited by U.S. mobile users in May showed the same dominance by e-mail and search sites. Nielsen found that Yahoo! Mail by itself drew 14.2 million mobile U.S. users—or one-third of the total Web user base. What’s more, those users visited the site 21 times a month, on average, and spent 10 minutes per visit.

Google Search was the second most popular destination on the mobile Web for that month, with 9.1 million unique U.S. visitors. MSN Hotmail got 7.9 million unique. Gmail had 7.5 million, Google Maps 6.2 million, and AOL Email got 5.3 million.

The only mobile content sites that made Nielsen’s top ten for May 2008 were The Weather Channel (8.6 million) and ESPN (6.5 million). CNN News came in eleventh with 4 million unique visitors.

While these findings might be interesting to would-be mobile marketers—after all, utility sites such as mobile e-mail can carry advertising– Covey and his fellow researchers found evidence that mobile Web access may extend the reach of other high-traffic Web sites. By combining mobile data with results from Nielsen Online, they found that the 200 leading Web sites accessed both from PCs and mobile phones saw an average 13% increase in their total visitors over their PC-based traffic alone.

“Some categories see more lift than others,” Covey says. “Weather sites do a good job of crossing platforms and getting onto the mobile phone. Entertainment news also sees a lift. That’s a little snacky: People who may not go onto TMZ.net on their work or home PC may feel they can sit on the subway and read about Paris Hilton without guilt over their phones.”

Covey points out, however, that what worked with pioneer users of the mobile Internet might not satisfy the next-wave mainstream audience. “Early adopters often put up with an off-deck mobile Web experience that wasn’t ideal,” he says. “If you were on a clamshell phone that let you type in a URL and have a very awkward experience with your favorite Web site, you might still do that to feel like you were on the edge.”

But broader audiences will expect a smoother mobile Web experience, and Web site operators should make sure they offer one with sites that work well (or at least properly) on the third screen.

“When you’ve got the seamless experience and cheaper prices of the 3G iPhone, the mobile Internet is no longer an early-adopter thing,” Covey points out.

Speaking of Apple’s Great Mobile Hope—in fact, speaking on the first work day after its July 15 launch–Covey says mobile marketers and carriers alike should at least be grateful for the amount of buzz the latest iPhone is creating around their Web channel.

“The iPhone today is just 1% of all mobile devices, although it’s used by a higher proportion of mobile Internet users,” he says. “But it’s attracted important attention to advanced-capability phones and improving network speeds. It’s probably been the biggest evangelist for the mobile media market to date. We’ll see more competitive devices in the coming year that play off the iPhone’s success and really build out the market.”