http://cdp.sagepub.com/content/16/3/128.short

If you don't want to click on it - here is the gist

Moreover, in-vehicle conversations do not interfere with driving as much as cell-phone conversations do, because drivers are better able to synchronize the processing demands of driving with in-vehicle conversations than with cell-phone conversations.

and another

http://cellphonefreedriving.ca/media...istraction.pdf

when controlling for driving difficulty and time
on task, cell phone drivers may actually exhibit greater impairments (i.e., more accidents and
less responsive driving behavior) than legally intoxicated drivers. These data also call into
question driving regulations that prohibit hand-held cell phones and permit hands-free cell
phones, because no significant differences were found in the impairments to driving caused by
these two modes of cellular communication.

and finally

http://dl.acm.org/citation.cfm?id=506599 (click on the PSU PDF)

People sometimes ask why cell phone conversation is
risky, in that conversation with passengers in the car does
not seem to cause accidents. We believe passengers
moderate their speech based on their observation of
current driving conditions. For example, most people
would stop talking to a driver who is passing a truck on a
two-lane mountain road after dark