I think cell phone conversations are more distracting than "in person" conversations because your attention is not on your physical environment - it's split. If you're having a conversation with a person in the passenger seat, you're both in the car, on the road, and your consciousness (while distracted by a conversation) is still present in your situation. A conversation over the phone kind of produces that same "absence" that you get when you space out for a while and notice that you're five miles down the road and haven't really been aware of your surroundings for a while.
I think eating or drinking is probably more dangerous just because of the mechanics involved, and I think I remember reading somewhere that that was true. I'll go hunting for references...
edit
Okay, here's what I found:
http://www.hsrc.unc.edu/pubinfo/dist...ion_topten.htm
The Highway Safety Research Center at UNC studied accidents caused by distractions. It looks like distractions outside the car are responsible for around 29% of crashes caused by inattention, while cell phones were responsible for only about 1%. Adjusting your radio is about 8 times more likely to cause you to crash than using a cell phone.