Twitter is. The other half don't think much of it as a source of news.
A new report by the First Amendment Center includes some questions about social networking, including Twitter (scroll to the bottom for these questions).
Three percent see Twitter as "very reliable" and 14 percent as "somewhat reliable" as news source. Thirty-four percent see is as unreliable (49 percent have no opinion, meaning to them Twitter is something birds do, not people).
Interesting factoid: African-Americans (11 percent) and Hispanics (10 percent) are more likely than whites (1 percent) to say Twitter is a very reliable news source. This might impress you if it wasn't for the small number of these folks in the overall sample (10 percent of the sample black, 5 percent Hispanic). The margins of error here are big enough to drive a truck through, making any interpretation of these numbers tricky. But it is suggestive. A racial/ethnic oversample might uncover some interesting trends, though off the top of my head I can't see any reason why one group would trust or distrust Twitter more.