I have an Acer Liquid Metal with Froyo and an Alpine Car Stereo (bti 114). I can use the stereo as headset perfectly and I've noticed that I can also hear the phone notifications when using it. Can I stream phone mp3s to my car audio? Any detailed information is very welcome :)
The stereo supports bluetooth hands-free but not bluetooth audio, which is used to stream music, so no, this is not possible. For more information see Bluetooth profile on Wikipedia and compare Hands-free Profile (which the stereo supports) and A2DP (which it doesn't).
Theoretically, you could stream music through the HFP but the quality would be terrible, like listening to music in a phone call.