I am seeing the same thing. I was running stock or blur based for a while and am now running CM7.2. The CM7 car dock app will not play anything out the USB/wired audio output. The Car Dock Ultra will play out the wired output upon insertion, but a call hangup does not reset the audio to wired from bluetooth as you noted, it goes to the phone speaker.
If you use bluetooth (why?) for audio playback, it does return to the last audio playing.
It is annoying.