The iPad doesn't have amplification, nor DVD playback, radio, nor video input. So right there you're limited. You have to have the higher end unit to have GPS.
There is no easy way to ingrate the steering wheel controls to the unit without someone licensing the connector and building the unit, in which case they'd might as well add the built in amp, radio (hd and satellite), video input (no API for loads of this in iOS 4.x).
This is not to even mention that getting the iPad to fit in the dash of many cars would be almost impossible. And the unit would need to be removable for syncing
