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T-Mobile USA inked a deal today with Yahoo for its oneSearch customizable home page and new shopping store, web2go, available across all its mobile handsets. While the carrier has embraced Google for its recently launched G1 smartphone based on the Android operating system, it appears to be tapping search competitor Yahoo for everything else.

“[The deal] was probably baked in,” said Bill Ho, wireless services analyst for Current Analysis. “It’s good for Yahoo, because Yahoo is going through bad times. If I were a wireless provider, I would want to play my vendors against each other, so nobody feels complacent. While these sweetheart deals are nice, I would always want to not have anyone feel complacent and have too much power.”

Yahoo’s relationship with T-Mobile already extends to 10 European countries added this past year with ad-search revenue-sharing deals. The agreement with T-Mobile USA will give the search provider access to 105 million US customers when coupled with an existing relationship with AT&T. Number-three US wireless carrier Sprint relies on a Google partnership for search, while Verizon still hasn’t committed to a search provider.

Yahoo has been battling it out with both Google and Microsoft for the developing mobile search market this past year, but lately the tides have turned against Yahoo. Yesterday, CEO Jerry Yang said he would leave the company, and today Microsoft chairman Steve Ballmer said that while he had no intent to revisit plans to purchase Yahoo, he is interested in revisiting a search partnership. Yahoo, in the midst of its rollout of a 3G network, has also attempted a partnership with Google, but it fell apart due to anti-trust concerns.

The Yahoo oneSearch homepage on T-Mobile lets consumers seek out mobile-specific data through voice queries or text-based search for relevant results as well as a catalogue of downloadable ringtones, games and wallpapers. The home page and search results are customized with a user’s preference in news, information and content. Details of the agreement were not disclosed, except that Yahoo would share ad revenue with T-Mobile. Ho said that from the carrier’s perspective, the portal and especially the voice -powered search came down to the need to drive mobile data usage.