Lack of BT voice dialing support is an Android issue, not strictly Droid, but there is a workaround:

An app called "Voice Dialer HF" is activated by pressing the camera button (don't worry, you can still use it for the camera) - something most can do without having to look at the phone. Once pressed, it prompts you through for voice commands.

Until the phone starts dialing, the prompts and your replies are going through the phone's microphone and speaker. They then switch you to the headset.

However, there are a few headsets that can do all the prompts and replies through the headset, notably the BlueAnt Q1. On these, the prompts are built into the headset and are sent to the phone. Scroll down to post 39 in this thread for a discussion of how it works.