"Water is fluid, soft, and yielding. But water will wear away rock, which is rigid and cannot yield..." So I say, you can try to manage the flow instead of stopping it altogether.
I say make a law such that when plugged in to the car port or charger, cellphones are given bluetooth-like functionality (mp3s & calls through the car speakers, etc), while not allowing texting (or a yearly changing list of functions that dont restrict the user *too* much). I hear the Verizon Droid seems to have the right idea of some kind of "driving mode" vs standard. With the coming wave of smartphone usage, people will really *want* to charge their phones while driving. And if a cop or camera see you not charging your phone, you get a costly ticket. It should change people's behavior like seat belts. Granted, its much harder to see the offense, so it will not be adopted as quickly, depending on how stiffly the laws are enforced, but it will change a percentage of people's behaviors (I'm thinking maybe 5%/yr topping off at 80% when everyone has smartphones). That percentage of change will save a percentage of lives, which is better than nothing, but at the same time not troubling people too much.
I, for example, am a habitual phone user in the car. Even though I know its dangerous, I just can't help myself - the smartphone is too damn useful & fun to use. I haven't been close to an accident yet, but if I were to get in one, I'd probably change, but a law like this would help push me a little harder. And if it forces phone & car companies to add cooperative, standards-forming functionality, it will keep me from getting too pissed off to complain to my congressman against it. And thats exactly what would be required to get any law passed at all.