A study by USC Annenberg School for Communication suggests that "Articles returned by Google News tend to be significantly more biased in one direction or the other than articles from Yahoo News." And that non-traditional news sources are a cause of that bias. Some background:
Google News, still in beta three and a half years after its launch, tracks the top stories on some 4,500 English-language news sites, updating its index roughly every 15 minutes. The ability to effectively search this huge collection of timely information has helped make Google News one of the Internet’s most popular news portals, drawing about 5.9 million visitors a month... Ranking news stories based on some measure of quality may be a step in the right direction, but to maintain its credibility, Google News needs transparency – both in its selection criteria and its list of sources.