It doesn't make the research useless. You can find plenty of recent research that any conversation with someone not in the vehicle -- i.e., someone who doesn't know when to shut up when road conditions get interesting -- raises accident risk considerably, whether the phone is hands-free or not. I've seen stats indicating that having kids in a car where the only adult is the driver shows the same tendency, although not the same degree.
You can Google around for the research. Someone earlier on this slashdot thread found this paper, for example. The exact level of risk varies between studies, but all of them are considerable. That's why studies of cell phone use have begun including hands-free use.
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