Gamin has apps for the iphone to make it act like their GPS.
Problem is it sucks the **** out of the battery.
http://www8.garmin.com/apps/

I have been thinking about getting this set up because they have a piggyback battery. It also works with ANT devices alowing you to have it run cadence speed and heart rate sensors if you like that sort of thing.
http://www.wahoofitness.com/