The other side of this coin, though, is that having just the maps app with GPS capability has saved me many times from being horribly lost to being able to find my way again (admittedly, this has only been on the iPhone up till now) . Even though Maps doesn't provide specific turn-by-turn, the blue dot will track you in near-real-time, which I've found is pretty close to a full turn-by-turn app, and for free.
But ya gotta have the 3G iPad for it to work.
(One last thought: TomTom does have a cradle with its own GPS capability for the iPhone, so it's possible they could either come out with a cradle for the iPad, thus providing WiFi only models with that functionality, or, it might be possible to use the TomTom existing cradle for the iPhone and attach some kind of connecting cable. You don't need to buy the TomTom application to use their cradle and get their added GPS function, at least that's my understanding.)