When I set up Ubuntu 8.04 on the D600 (with an ATI display controller), I ran into some problems. Running two displays was wonky. Getting video over to the second display was wonky. MythTV would crash when attempting to play MPEG-2 videos. Getting audio out to the USB speakers was odd. I never could get my Bluetooth tablet to work. Recently, new divx .avi files would not play (so I had to use Windows to play them).
8.10 beta has its own oddness, but...
Multiple display support with Xinerama works beautifully.
PulseAudio works beautifully.
All my divx .avi files play fine.
MythTV plays all my files fine.
The Bluetooth tablet works great.
All in all, a major improvement.