Google, Bing and Yahoo are all showing twitter content in their search results now but Google have taken things a step further with Google Social Search from Google Labs.
Matt Cutts explains how it works:
Note the emphasis that Matt places on the fact that all this data is already publically available!
Basically Google rounds up your social networking activity (such as Twitter, friend feed, blog subscriptions, picassa and so on) and spiders the hell out of it so it can return stuff from your friends and social connections in the search results pages of Google, if you are signed in to your Google account. Only you will see stuff from your network.