If you search this forum, you will find detailed explanations of exactly what Bluetooth limitations exist and why. However, to summarize, Droid will work with Bluetooth for handsfree dialing.
1) If your Stereo manages it's own contacts address book, you can import your contacts from the Droid, and it will work fine without any limitations, since the Stereo manages the voice dialing functinality, and simply tells the Droid what number to dial.
2) If you have a stereo that relies on the functionality of the Droid to do voice dialing, you can still do hands-free voice dialing with 3rd Party Apps (Voice Dialer HF). However, ANY solution that relies on the Droid for Voice Dialing will also use the Droid Mic - Not the Bluetooth Device Mic - for dialing. For this solution to work, the Droid must be mounted near the caller, since is requires the use of the built in mic to complete Voice Dialing.
Once the call is connected it will route all communication (including mic) over Bluetooth.