The study, which is to appear in The British Medical Journal today, is the first of its kind to use actual crash data and cellphone records to show a link between talking on the phone and being seriously injured in an accident.Read more about the report at NY Times
It is also the first to conclude definitively outside of a laboratory setting that holding a phone to the ear or talking through a hands-free device pose the same risks.
Hands-free kits don't improve road safety

Governments push cell phone users to use hands-free kits in cars but a study of Australian drivers concludes that hands-free devices don't improve safety. The study claims that no matter how, making phone calls during driving fourfolds the risk to be involved in a serious crash.