Indeed magicmick. It's not holding the phone, it is the attention being diverted from the act of driving to the conversation. Often I am behind a car that is weaving around and driving extremely slowly and I can see the driver is heavily engrossed in a conversation with the passenger, taking their eyes off the road to look at the passenger during the conversation. The act of driving is maybe 4th or 5th in the list of things to concentrate on at that point.
I tried using hands free for my phone and after a great deal of swearing I managed to bluetooth my blackberry to my car. Complete waste of time. When giving my driving the necessary concentration all I could manage on the phone was the odd Neanderthal grunt.