I have not actually used this, so I can't vouch for it, but, the newer, higher end, garmin portable navigation units do what you want.
They can talk to your phone using Bluetooth
The microphone is in the navigation unit
It transmits the voice on FM so you listen to the phone through the car's radio speakers.
I believe the nuvi 660 does this, probably the other ones too.
I suspect it is not quite as nice as a built in, because I think, you will need to change the radio station.
(and you get navigation for free
BTW -- OnStar (more specifically, GM's lack of a Bluetooth car kits because they want to sell you OnStar so that you pay them every month) is why this "buy American nut" won't buy GM cars. My current car is a Chrysler with U-connect (Chrysler's built in bluetooth car kit). 2008 Fords will have "Sync My Ride" which is even better.