DaveB I think you and I do (or would) use a tablet in very similar ways. As mentioned I don't travel across the globe, mostly within 200 miles of home which reduces the amount of info I'd need to access any given time. Map-wise what would be most useful would be zoomable maps, showing a general route getting me near a destination. Once close to the final destination zooming in to see individual street names etc would be out-of-this-world beneficial and the main reason something GPS-like is of interest to me now.
Theft-wise I'm fairly okay with such a device being left in the work truck while its not in use---there's virtually no way to see inside while parked overnight, alarmed on-board too. When/if parked temporarily (Loew's etc etc) pawn shop gems are always hidden from view. In fact knowing me such a device would constantly be left in the truck, power adapter or charger within quick reach when needed.
Viva's recommendations about the 3g-enabled iPads is very helpful too. When I make the pilgrimage to the local mega-computer store zeroing in on a few specific models will keep me from being overwhelmed by the millions of choices I know I'll face.
All in all I think the dedicated GPS's like Garmin etc have a bit more than I need or want----you guys have been great teaching me about what they typically include. POI's and such don't really appeal as I'm usually on the road only for business, not a lot of sight seeing then.
If the graphics and info display aren't all that impressive that's another reason to more carefully consider the iPad (or its kin) approach.