I think that the main difference is that one is temporary and optional, the other lasts through the duration of the drive (and may get worse on the way). I ride drunk all the time, but I know that I am impaired a lot of the time. I wouldn't drive a car like that.
When you are on the phone, it may only be 30 seconds of distraction. When on the phone without a headseat or speakerphone, a hand is usually taken off of the wheel, which limits motor control of the car, too. This may explain the correlation in accidents for cell/vs alcohol....
There have been a lot of inovations integrating cell phones into cars to allow the person to concetrate all of their physical thought on driving, while holding a conversation. I know that people have talked in cars since cars were first made - it's not that different than talking to someone in the passenger seat.
I don't really know what my point was, but I've been typing for awhile, so I'm just going to post it...