3G+Wifi Model:
3G (AT&T)
GPS
WIFI (AT&T-tethered, verizon mifi, sprint overdrive, etc...)
I'm honestly surprised at the number of users who claim the GPS function of the iPad is what makes the thing so useful. I just cant imagine using a GPS of that size in my vehicle. If you need something with that large of a form factor to navigate, I really wonder why you are holding a drivers license in the first place.
The only relevant difference between 3G and WiFi-only iPads is, of course, the existence of 3G.