I've been arguing about this with RogersMary for some time.
To be clear for those saying BT dialing is only available in 2.2, previous
to the E911 update that brought Sense UI those of us with the HTC Magic WERE able to start the voice-dialer by pressing the button on the BT headset. This is how I did it while driving, and the reason I was so mad when the E911 update came out and that functionality was missing. (I can dig up the threads if anyone cares).
Based on what I've read around the intertubes, apparently the ability to start the Voice-Dialer from the BT headset button was fairly unique to the Rogers build of the 1.5 ROM. So, my guess would be that when they discovered the 911 bug they had to get something out ASAP, and took an existing fixed 1.5 ROM from somewhere (without
the BT dialing) and made the Rogers modifications to that. I cannot confirm that however.
Mary is on vacation until later this week, and I was only able to confirm with her shortly before she left that the BT dialing was STILL missing in the 2.1 ROM, so when she gets back I'll ask her about it again.
If there is anyone out there who still has a PRE-911 Rogers HTC Magic with the stock ROM on it, you will see that you CAN start the Voice Dialer app by pressing the button on a BT headset (I was using a Motorola headset).
For now, I am using one-touch dialing icons on my home screen with NO other icons on that screen, that way I can safely press just one button on the screen (which is held in a cradle attached to the dash) to initiate a call. I also use the app "Carmode". Run Carmode when you get in the car and all calls automatically go to the Speakerphone, this way you don't even have to bother with a BT headset.