The problem with the Wifi version is that it doesn't have that GPS chip that the 3g does.
While I plan to tether my iPad to my iPhone for 3g, and initially thought I'd just get the Wifi version, because it doesn't have the chip - I'm holding out for the 3g. And I can't wait to have it in my car.
That said, we do have a wifi version here in the house and WOW! It's wonderful.