We should probably round up some people who are on the MotoDev team. (Technically, I'm one of them, except I haven't actually done any development work since joining.)
Java is all the rage these days, especially in mobile phones, so it's possible that a Java application can be developed to utilise the phone's Bluetooth module to connect to the car PC. It will then have to bridge the two network adapters: GPRS/3G and the Bluetooth PAN.
Admittedly, it would be a lot easier for people who have a Windows-based phone because there is an Internet sharing application built into Windows Mobile and you can set it to share through USB or Bluetooth. This is how I plan to access the internet with my car PC ... once I actually build the car PC and once I change my rate plan