Check out bluephone elite.
I have recently installed a MacMini(using Carnetix) in my car. I also enabled the Address Book to work with my Sony Ericsson P990i. The 2 work well. My MacMini streams music to my alpine Head Unit and I use Front Row to navigate the beauty. One of the things that has struck me is that the Mac Mini has a Mic and stereo jack behind. One could attach a Microphone to the Mac Mini....to answer mobile phone calls and also hear the caller thru the car's audio since the mac mini is connected to the car's audio already. I believe somebody good with bluetooth skilss could create bridge to handle calls thru the mac mini and make it truly HANDSFREE!
If this idea has not found a place I am sure it is in the works. If somebody knows anything about this or has prior knowledge, please do share it with me.
I have a similar setup in the Subaru Outback. I had to build a powered mic using a kit from J-car and it works brilliantly on bluephone. This is one app that is worth buying. Heavily customisable and easy to use. When I get a call, iTUnes PAUSES (why, oh why do most apps just mute?!?!) and the sound comes out crystal clear through the car speakers. The other party has a good clear sound from the mic I built in to the central spot array overhead, and when I ma finished, bluephone hangs me up and the music comes back on. I never have to look at or touch a thing.
Texts are cool and when I get motivated, I'll get the screen reader to read them to me.
It's not that hard if you are prepared to spend an hour or so with a soldering iron and run the cables.