T-Mobile announced it has chosen Yahoo to power its new web2go portal, which will have T-Mobile subscribers who use the Internet use the Yahoo-powered service by default.

T-Mobile subscribers will be able to browse the Internet along with search through the T-Mobile content catalog of ringtones and wallpaper.  Yahoo will provide advertising on all search result pages, with both companies sharing advertising revenue.  Revenue sharing and other contract details were not announced by either company.

Yahoo has been locked in a fierce battle with Google and Microsoft in PC and mobile searches, though Google has dominated Yahoo as of late.  In an effort to undercut Google before it’s able to begin to dominate mobile search, Yahoo is carefully working to get its OneSearch service on as many phones as possible.  

T-Mobile has 32.1 million subscribers in the U.S., and has been trying to turn more of them into Web browsers, so the company can sell them data plans.

Deals with T-Mobile and AT&T will give Yahoo a combined network of 105 million subscribers in the U.S. who use Yahoo search portals.  Furthermore, Yahoo and Deutshe Telekom already have been working together in 10 European nations, with both companies sharing ad search revenue.

ComScore M:Metrics reported 60 percent of 17.6 million mobile Web users in September used Google, while 36 percent elected to use Yahoo.  

Yahoo also has multiple agreements with countries such as India, Vietnam and the Philippines -- even though the deals haven't provided much revenue yet, the Sunnyvale-based company hopes wider mobile phone adoption in those countries will help increase mobile advertising as well.

The announcement came hours after Microsoft CEO Steve Ballmer announced his company was no longer interested in buying all of Yahoo, but a search deal was still possible.