I can't tell you how many times I have gotten into this discussion/debate/argument with people. I think many people have finally come to the realization that talking while driving is just as bad with a hands-free device because it is the mental distraction of the social interaction that causes most of the problem. But they don't see it with texting because you don't have that social link. With no social and no hands, it can't be as distracting can it? Yes, actually, it can.
To HF practitioners, it is a no-brainer. But it is so counter-intuitive to the typical driver that they refuse to believe it. Great example of motivated reasoning (finding reasons to believe what makes your life easier) and commitment bias (finding reasons to justify what you have done in the past).
People out there. Texting while driving doubles your reaction time to important things like a car cutting you off on the highway or a small child running into the road to get her ball. Hands-free and hands-on are both double. No difference. If you hands free text while driving you are going to kill someone someday. Perhaps yourself. Please stop.
Luckily, David Pogue has finally been convinced. He has some clout because he has been the tech writer for the New York Times for 14 years and was a denier for most of them. (real time update - he just quit the NYT today and moved to Yahoo. But he switched sides on the handsfree texting first).