Follow through on some of the links for even more information:
http://www.nsc.org/issues/idrive/inincell.htm
3 paragraphs I find rather interesting:
The principal findings for this experiment are that: (a) SPs that engaged in cell phone conversations missed twice as many simulated traffic signals as when they were not talking on the cell phone, (b) SPs took longer to react to those signals that they did detect, and (c) these deficits were equivalent for both hand-held and hands-free cell phone users.
In sum, we found that conversing on either a hand-held or hands-free cell phone led to significant decrements in simulated driving performance. We suggest that the cellular phone use disrupts performance by diverting attention to an engaging cognitive context other than the one immediately associated with driving.
Our data suggest that legislative initiatives that restrict hand-held devices but permit hands-free devices are not likely to reduce interference from the phone conversation, because the interference is, in this case, due to central attentional processes.
So basically they conclude that any cell phone usage hand held or hands-free creates issues.