There are a number of iPad topo/mapping programs (apps bleh) out there, some of which allow 'offline' storage of the maps, so from a data standpoint, you have options for offline use.
As for GPS, I think Haven said there is no GPS in the WiFi version, and the 3G GPS is assisted, which some people have said works outside service, while others have had issues (user or computer, idk)...we'll just have to wait and see...
From what I can tell, no aftermarket GPSs out for iPad yet, but I wonder if some of the iPhone/iTouch will work? That's something someone is going to have to try and let us know (Haven?)...I do think that accessories will be much more expensive because they are 'iPad' just like Hummer accessories are more expensive just because of the name...we'll all just have to live with that...
I think the iPad will be great for a lot of people, ok for others, and some will hate it because it's the Yankees/Duke/etc...