I have a proof of concept code that works_for_me connecting to Bluetooth GPS (or any Bluetooth device with a COM Port profile) and A2DP simultaneously (i.e. you can use BT GPS while listening to your music in the car).
It works thanks to the changes in the iPhone BT stack made in 4.0, so you need to have 4.0 Beta installed (and jailbroken). You also need to have a Bluetooth GPS (obviously).
Please PM if you meet the criteria.