The real question of banning cell phone usage in automobiles has nothing to do with safety or good intentions; rather, at what point do we stop allowing the government to overstep their boundaries? What occurs in a vehicle is ultimately no one else's business as it is private property; thus, the only judgment that can be enforced is how the car appears externally.
In such a case, the issue has less to do with a phone and more to do with reckless driving in general. Sure, it would be great to crack down on reckless drivers. It would also be prudent to crack down across the board on factors of driver error or incompetence. The trouble has never been a lack of desire. It has been a lack of funding and support that prevents more detailed enforcement. You can also vote to raise your taxes, but it doesn't sound so appealing then does it?
Where do you really think it will stop with the laws over private property? Perhaps, no children should be allowed in a vehicle under the age of ten unless they have a separate monitor. If we push the line between public and private, that also brings smoking into play as it is banned publicly now in most other situations. What you choose to wear in your car might also be checked in accordance with public laws of decency. The sky is the limit when you start removing your rights to private property.
Radios, GPS units, and the entire spectrum of other electronic devices will likely find their way eventually into the laws governing cell phones. What matters more, however, is that police can barely keep up with current enforcement. This means that other laws that already get ignored or under enforced will get even less attention. At the same time, attention will likely be very slim when it comes to cell phone usage. In the end, there is really no substantial gain and a lot of potential sacrafice.
What it all boils down to is do you value more freedom or safety? Both come at a cost and usually one is part of the cost of the other. The more freedom that you sacrifice, the safer you will be as long as you adhere to the new system of laws. It may sound appealing, but visit any other country with harsh laws limiting personal and private activities before you are so quick to make this judgment. You will find many unexpected costs and sacrifices when selling your freedom.
What we should do and what we have the right to do are two very different things. If you question the very freedom of our public property then you are opening the debate on all of these issues.