There are several oilfield companies that now ban use of cellphones in company vehicles - get caught, get fired and in one case I know, they define the commute to/from work in your personal vehicle as "company travel".
Now that more and more phones have GPS embedded in them it's really a simple step... if the phone is moving more than 4 mph (walking speed) just have it disable outgoing calls and force incoming to the voicemail. The only real pain would be if you are on a boat or train on a commute - but then I wouldn't have to listen to your argument with your boss, customer or girlfriend when you start talking loud enough to not need the phone in the first place.
I realize that people are so much more important today, or so much more insecure that they just HAVE to be instantly available, but... how did we ever survive for the last 100 years?
Rob