"Bias is clearly in the eye of the beholder"
Bias, while most often perceived as subjective, is actually objective when an analysis is done word by word and phrase by phrase and the stories covered compared to the stories available. There is no question that the major networks are biased, but there is some doubt if they will ever admit to it.
If you want to read an objective academic study about media bias, see:
http://www.cbrss.harvard.edu/events/ppe/papers/Tim%20Groseclose%20Media%20Bias%20Paper.pdf
Surprise, the study showed CBS to be the most liberally biased news source.