I did test the AD2P hack described here in the forum with a bluetooth headset and the audio quality is great.
Only disadvantage is, that it's a pain in the *** to set this up and also the 50% CPU usage is not so nice. Hopefully Nokia will add official and better integrated support for AD2P in the future.

I'm planning to buy a new car stereo (Sony MEX BT 5000) in the not so far future. I'll give some feedback here, how the audio quality is when connecting my N800 via bluetooth to the MEX BT 5000.