From the Business Week article, The World According to Ballmer

Clearly alluding to Microsoft's key Internet search rival, Ballmer said: "The hottest company right now -- the one nobody thinks can do any wrong -- may just be a one-hit wonder."

Actually Google is already at least a two hit wonder; AdSense/Adwords and the Google search engine. Given that revenues from AdSense are growing to match those from the Google website, it seems inevitable that in a few years it'll be more important to Google that they are the #1 ad provider on the Web not whether they are the #1 search engine.