Apparently it's true, according to Tim Groseclose and Jeffrey Milyo (2005, Quarterly Journal of Economics). I just came across the article on a random troll of the internet. Here's a quotation:
Our results show a strong liberal bias: all of the news outlets we examine, except Fox News’ Special Report and the Washington Times, received scores to the left of the average member of Congress. Consistent with claims made by conservative critics, CBS Evening News and the New York Times received scores far to the left of center. The most centrist media outlets were PBS NewsHour, CNN’s Newsnight, and ABC’s Good Morning America; among print outlets, USA Today was closest to the center.One thing I found puzzling about the study is that it ranked the WSJ almost as far to the LEFT as Ted Kennedy and farther to the left than the NY times (by a significant margin), based on the groups and studies they cite. I'm not sure I buy that part of it, but interesting read nonetheless.