(from
http://www.themonkeycage.org/2010/03/has_health_care_reform_enraged.html)
Of course you'd have to look at other major news events to be more scientific, but the basic point I think is that even though Fox has the top ratings overall, people who aren't political junkies but will only turn on cable news when real news is actually being made in real time still tend towards CNN.
(Note that the chart is relativized to last weekend's ratings, so FOX's baseline is higher, but that's the point: to look at viewers who aren't political junkies, you have to abstract away from who's watching cable news on an ordinary Sunday night. Those who are into this sort of thing can follow the link for raw Nielsen numbers).