Although the United States lags other markets in terms of mobile, it is the leader in mobile Internet penetration, with 40 million subscribers, or 15.6 percent of users, accessing the Web from their handsets.

This was the key finding of a study by Nielsen Mobile that examined the global mobile Internet market. Of the 16 countries tracked, the U.S. leads in mobile Internet penetration, followed by Britain with 12.9 percent and Italy at 11.9 percent.

"A number of moving pieces have come together to bring mobile Internet a critical mass," said Nic Covey, Chicago-based director of insights at Nielsen Mobile.

"Factors such as devices, network speeds, audience interest and content have arrived at a point where mobile Internet marketing should be a very real part of the consideration set for media planners," he said.

Critical mass means mobile Internet now delivers reach comparable to other major marketing mediums such as broadcast television shows and major print publications, Mr. Covey said.

Nielsen Mobile's study found that Motorola Razr phones are the most-used handsets among U.S. mobile Internet users, while Nokia handsets lead the market in Europe and Asia.

In addition, unlimited data packages are increasingly popular with mobile Internet users. An estimated 14 percent of U.S. subscribers access the mobile Internet with an unlimited data package and 50 percent of data users said they prefer the unlimited pricing model.

The study also found that 3G networks drive user satisfaction with mobile Internet. These networks improve data throughput speeds as much as six times, compared with 2G and 2.5G networks.

"I'm glad we're able to quantify the extent to which 3G networks improve the mobile Internet experience," Mr. Covey said.

"Our research shows that the network speed is the leading driver of satisfaction with the mobile Internet, so it's encouraging to see results from our drive-testing around the U.S. that show data points throughputs that are six times faster with 3G," he said.

Advertising is becoming an accepted and common part of the mobile Internet experience. In fact, 26 percent of mobile Internet users view ads while using the mobile Internet.

Additionally, mobile Internet users are more receptive to mobile advertising than average data users.

Among other findings, Nielsen Mobile identified the top 10 U.S. mobile Web sites as of May 2008. Yahoo Mail, with 14 million unique monthly visitors, is the leading mobile Internet channel.

"The [mobile] market is still emerging and consumers are just warming to the idea of mobile marketing," Mr. Covey said. "Encouragingly, consumers expect to see more of it in the future. They are open to advertising as a tradeoff for things and mobile Internet users, compared to all users, are more likely to be open to mobile advertising.

"Still, it can't be overstated that marketers need to respect the very personal nature of this medium while also seeking new, value-added ways to engage with consumers over their phones," he said.