If the iPhone will only be allowed to run one app at a time, it would not be possible to run a GPS application and answer the phone while you are en route to your destination.
Integrated like the iPod would work, but would be pointless. GPS is no big deal to me either.
When you receive a call, the audio stops and then restarts after your phone call is through. So, if you're on the Autobahn, and you miss your turn... whoops.