The conventional wisdom is that the next field where the search engine wars will be waged is mobile devices. The theory behind that is that PC users have already decided what search company they want to use. In about 70% of the cases in the US, that is Google (NASDAQ: GOOG).

With the computer market pretty much gone, if Yahoo! (NASDAQ: YHOO) wants to pick up any market share from Google, it has to aim to make deals with handset companies and cellular service providers. It is going down that path, but the success of the move is likely to be modest.

According to Reuters, "Yahoo Inc announced an expansion of its mobile Web portals to T-mobile, so its smart phone users who get data will have Yahoo search by default." Yahoo! also has a deal with AT&T (NYSE:T). The partnerships give the carriers a piece of the search advertising from the mobile service.

Unfortunately, the new deal with T-Mobile will probably not work well. Being the default search engine does not mean much. Almost every person who has a cellphone knows how to set the mobile browser to use Google. In most cases, PCs come with a default browser, and if it is not Google a lot of consumers simply change the setting.

Google does not do well on the PC because it is set up as the first option by the manufacturer. It does well because it is the most effective search engine. People getting T-Mobile phones already know that.

Douglas A. McIntyre is an editor at 24/7 Wall St.