What about the possibility that there were more positive articles about Obama because he was the better candidate, more interesting, and just generally led reporters to be more positive? Maybe instead of "bias" that's just "truth". McCain was boring and unpleasant.
After reading this post from 538 it occurred to me: reporters are actually putting a lot of effort into supporting the facade that the McCain campaign isn't depressing and lifeless. If they let through the truth of it sometimes, that's not bias, that's just doing some decent reporting from time to time.