Well, I got my USB serial cable to tether my phone to my PC and saw a couple of improvements:
1. I now get 38kbit/sec instead of 20kbit/s
2. I can now receive calls whilst connected to the internet via GPRS.
The latter is a big win. So now I need to work out how to use the PC's microphone and speakers to make phone calls and I'm largely set when I finally have my PC ready to go into the car. Tethered is not ideal - I'd rather have it all just work off bluetooth but I can make a phone stand with the serial cable embedded in it - using it like that won't be much of a hardship as long as I can get it to use the PC's microphone/speakers...