My discussion with cell phone people (years ago) led me to believe that security was at issue.
I beleive Marc, in that the US cell phone companies are interested in charging more for data services.
The units sold at sparkfun.com web site state:
"You will need to provide a SIM card. Pull the SIM card from your phone, insert it into the module, and viola, you are making calls under your cellular account. Works with all providers currently supporting SIM cards (AT&T, T-Mobile, and Cingular)."
So, the bottom line here is that no-one in the US sells cheap off-the-shelf cell phones with a simple serial interface ( like everywhere else in the world).
Donald
PS: the cars have a modules like the GM862 Cellular Module.