Have a TT roadster on its way and want to change out the head unit on the premium sound system to something that will take a GPS subsystem. Anybody tried anything like this and have some suggestions?

Have you checked out the Blaupunkt TravelPilot RNS 149 ? It's a combination radio, CD player, and GPS navigator (with voice assist) all in a single DIN.

I kind of want one myself....

Actually Yes,I did this last year with a Alpine CVA 1005, all you need to do is have a custom Din surround made and grind down the round knob on the face of the unit.Also note you will need a powered antenna adapter.The brain was mounted in the trunk of the TT (coupe).
Unfortunatley I had to remove this and replace my entire system back to stock for ESP recall.I have since sold the head unit but do have all the phoenix gold amps and a custom subbow with 2 12" solbarics.