Windows Mobile-based Phones
One can easily connect their car PC to the internet could with many Windows Mobile-based phones. There is an internet sharing application and you can select whether you want to share your GPRS/3G connection either via USB if your phone is connected to your car PC with a USB cable, or via Bluetooth if you have a Bluetooth-enabled car PC.
Bear in mind that such a setup will reduce the battery life of your phone because of the relatively high density of data with internet usage, as opposed to the lower density of phone calls/SMS. Also, the speed and bandwidth will be limited to those of your phone's connection, and then possibly further restricted by the connection you choose (USB or Bluetooth).
There is some more info about internet sharing on Windows Mobile on xda-developers.com. I'll post the thread links when I find them :)
Hope this helps. Perhaps it could be turned into a sticky thread.