Interesting idea. Out here in the 'burbs we don't have any parking ramps to keep track of levels, and most lots don't have #'s, just lots of big parking lots. I've seen another way of doing it.
Here is a compass keychain fob that someone makes.
An approch you could take on the iPhone/iPod would be to have a mode where there would be an arrow that could be rotated. A person would get out of their car, stand behind the car with the side of the iPhone/iPod parallel to the car and the top parallel to the building your going into. Then just rotate the arrow until it points at the door that you will be entering/exiting. When you come out hold the iPhone/iPod with the bottom parallel to the building and the arrow will point in the direction of your car. I've seen little keychain devices much simpler then the compass one that simply have a dial with an arrow on it.
Of course both modes could be used so when you get up to the parking ramp level you set in the first mode you can see when direction from the door your car is.
Attached is my quick mockup.