Google’s YouTube has rolled out technology to improve viewing its user-created videos on mobile devices.

Even though versions of the service have been available from Verizon’s VCast and on phones from Sprint, AT&T and Apple’s iPhone, they have offered only a small subset of the YouTube catalog.

Now YouTube has made most of its catalog of millions of videos available to hundreds of millions of mobile users with 3G handsets. The new service, at m.youtube.com, also offers personalization and community features, which the old services didn’t offer.

But the new service isn’t for everyone. First-time visitors to m.youtube.com are met with this message: “YouTube Mobile is a data intensive application. We highly recommend that you upgrade to an unlimited data plan with your mobile service provider to avoid additional charges.”

The Full YouTube Experience

“The mobile experience will be much closer to the Web experience,” Hunter Walk, a YouTube product manager, told The New York Times.

“It’s basically the full YouTube experience you can get on the desktop — on the phone,” Dwipal Desai, YouTube’s mobile-product manager, told Reuters. “We expect it to get fairly popular from our past experiences.”

While many phones will be able to access the new service, most Verizon phones will not, Desai said. There’s no word on when the situation might change for Verizon users.

‘Monetization’ Later

YouTube is also testing software that will allow mobile users to upload videos directly from their phones to the site, which would make many more videos available. Currently video has to be downloaded from the phone to a computer, and then uploaded to YouTube.com or another service.

There are no immediate plans to put advertising on the new service. “Right now we are focused on building a user base on alternative screens, and we’ll look at monetization in the future,” Desai told Reuters. That is…