Yahoo! has confirmed that now Yahoo! OneSearch is available to all US mobile web users. As it was a part of yahoo! Go interface its use was restricted to Yahoo! Go users. Now it is compatible with nearly 85% of mobile phone users with web search in USA
Yahoo! Mobile informs:
Finally, mobile search that really works! Introducing Yahoo! oneSearch, an amazingly better search experience that redefines search for the phone. It’s designed to give you instant answers—exactly what you need when you’re on the go. Yahoo! oneSearch understands the type of search you’re doing and optimizes the results accordingly—so you get the answers you need instantly with just one click, right there on the page. oneSearch includes more actual content in your initial results than any other search—all grouped by subject matter and relevance, so there’s no sea of links to wade through like with a PC search. It’s easy to read, scroll through, or drill down further if you want more details.
Loren Baker has an in-depth post on Yahoo! Mobile.