Check out the Parrot MK6000. It's a bluetooth handsfree kit with stereo audio streaming.
http://www.parrot.com/usa/products/b.../parrot-mk6000
http://ca.youtube.com/watch?v=x6fLKlnZZog
I haven't tried it yet, but have been looking at it for a long time. It's $149.00 USD plus installation, so much more expensive than the Radioshack/soldering method, but certainly a very elegant solution.
My understanding is that once you've paired it with your phone and synced your contacts, all you have to do is turn your bluetooth on when you get in your car and start your audio. I'm thinking Orb is going to be my weapon of choice, or you can go with streaming radio or local audio files (.mp4 and .aac are supported). The music then streams to your car stereo via bluetooth and will pause when you get an incoming call. The caller's voice is then routed through your car stereo speakers and your voice is picked up by the microphone included in the kit. Finish the call, the music starts again. Pretty slick. It also looks like you can also skip from track to track and adjust the volume with the MK6000's controls.
The only worry I have is that I will be using the the data radio and the bluetooth radio at the same time, with the voice radio thrown in there while on calls, so I imagine battery drain will be fairly massive. This will probably mean that my Diamond will have to be plugged into the charger at all times. Not really a big deal. That and the heat may very well transform said Diamond into a pool of molten plastic. Maybe that will finally convince Telus that they have a heat issue with this phone!
As I said before, I don't own this device nor have I ever used it, just sharing my research. Has anyone else had any first-hand experience with the kit that they could share?