This is another reason why cell phones should be banned when driving. It doesn't matter whether it's hands-free or not:

The researchers found that drivers who had used a mobile phone - either holding it to their ear or using a hands-free system - were 4.1 times more likely to have an accident in the 10 following minutes than if they had not made a call.

("Phoning while driving quadruples crash risk: study", AFP, Yahoo! News, Jul.11)