My idea is that cell phones should allow drivers to hit a button that would put an automatic one- or two-second delay in transmissions to and from the the speaker, like what used to be the case for international phone calls.
As long as people will use phones while driving, even hands-free versions can be very distracting. A two-second delay slows down the pace of the conversation, allowing the driver to increase the amount of attention paid to the road. It also provides a subtle hint to the person at the other end of the line that the driver needs to concentrate on the road, so the other person can also slow down the conversation, or even offer to call back later.
I read somewhere that children in a car are just as distracting as cell phones, but adults aren't - the implication was that adults understand when to allow a driver to concentrate on driving. Maybe this idea would also help.