In recent anouncements, Google and Bing have confirmed that they use information from social media to determine ranking. This includes Twitter and Facebook. This is a relatively new development for Google, although many had long suspected it would happen.
This video contains some insight from Google’s Matt Cutts: http://www.youtube.com/watch?v=ofhwPC-5Ub4&feature=youtube_gdata on “Does Google use data from social sites in ranking?”
Most interesting is that your stature on Twitter could help influence how a page ranks in web search.