Another vote here for the Iron Aces mount, and the MotionX GPS app.
The only problem (initially) was that you couldn't lock the orientation to landscape, so on the twisties, the iPhone was changing to portrait and back. I sent an email to the developer, and explained my problem. They now have a setting that you use to lock the orientation into landscape mode. Problem solved!
Any of you that use this approach can now shower me with your affection for getting this fixed... Small bills only...