MediaPost reports that now that 15.6% of U.S. mobile users actively surf the Internet, we've reached a critical mass for mobile Web marketing, according to a new study by Nielsen Mobile.

The report said that 14% of subscribers have signed up for unlimited Web surfing on their cell phones. Half say they prefer the unlimited pricing model—we think that number should be 100%, because anyone who signs up for MB-limited browsing usually gets a nasty phone bill at some point. That's because they didn't realize the phone was going out to the Web on its own and racking up data charges in the background (such as automatically checking for e-mail).

Anyway, the report said that the top U.S. mobile sites as of May were Yahoo Mail, with 14.2 million unique visitors, followed by Google Search with 9.1 million, and The Weather Channel with 8.6 million. More than a quarter (26%) of mobile Internet users view ads and "overall are more receptive to advertising than average mobile data users."