Despite its endless problems in search advertising, Yahoo is far and away the leader in the rapidly emerging carrier-controlled world of mobile search.

The Sunnyvale, California, company on Thursday said it inked a deal with T-Mobile USA that will make Yahoo’s oneSearch T-Mobile USA’s preferred search engine.

The deal gives Yahoo choice real estate on the phone screens of two of the United States’ top four mobile carriers.

In September, Yahoo got the nod from AT&T as that carrier’s choice to occupy prime real estate as the first search engine its subscribers will see after turning on their phones. (AT&T, Yahoo Link on MobileSearch)

Google has a similar deal with Sprint Nextel, and Verizon Wireless is reportedly in talks with MSN and Google. (Microsoft, Verizon Closeto Mobile Search Deal?, Microsoft,Verizon Close to Mobile Search Deal?)

The T-Mobile arrangement gives Yahoo more than 70 international carrier deals. Those deals give the search firm the potential to reach 850 million mobile subscribers, according to Yahoo spokesman Cory Pforzheimer.

Google has an estimated 50 carrier agreements.

In February, Google and Yahoo faced off over a mobile search deal with German carrier Deutsche Telekom, T-Mobile USA’s parent company. Yahoo won the deal as the preferred search engine in eleven European markets. Google was the incumbent.

“This is a great area of focus for Yahoo, and we believe we have a leadership position,” Mr. Pforzheimer said.

The mobile search market is still in its infancy. Carriers, particularly those in emerging markets, are still rolling out their mobile broadband services.

But mobile phones outnumber PCs two-to-one and that ratio is expected to grow to three-to-one by 2010 when mobile phones are expected to top 3 billion worldwide.

The carrier deals give search companies early access to mobile consumers, with the potential for them to generate ad revenue from search and other applications such as email.

In return the carriers get the benefit of attractive search algorithms designed to optimize ad revenues.