Just in case anyone else was interested as I looked all over the place to see if this is possible. It is....
I have tomtom6 running with my nokia 610 car kit and it works wonderfully. Obviously this is with the latest firmware on the N80. (Big thanks for u lot for instructions on updating this on the site!!)
It connects fine and hasnt dropped connection via bluetooth at either end so far. Only thing that doesnt work is the tomtom call the nearest POI near you. This doenst work as far as I can tell which I wasnt surprised as the remote sim function on the car kit doesnt actually recieve data from the phone like that. But I can accept incoming calls without tomtom shutting off, I can call out. Also recieving texts isnt a problem too.
I guess this will be the same for other fixed bluetooth kits in BMW & VW cars around.
Any questions I can try to help.