I think an iPad with external GPS makes good sense, primarily because an external GPS can be placed on the dashboard of a car or airplane and "see" more of the sky more clearly through non-metallic material.
An example of a non-aviation use:
My wife and I went on vacation to England in September and we took a wifi-only 32G iPad 2 along, external Dual Electronics Blue Tooth GPS, and Navigon MobileNavigator British Isles mapping software installed. It worked great in getting us around in a rented car. I've never driven from the right seat before in the left lane: decades of instinct made for a nervous drive initially (not to mention they never invented shoulders for a lot of their older roads; tall hedges abound that come right up to the edge of the roads.)
All in all, the Blue Tooth GPS worked well in the car with my wife handling the iPad. We've also used it in a C-172, and the rubber mat that Dual supplies does a very good job of keeping the GPS where you put it on the dash. By comparison, I'm not as confident of the Zaon MRX PCAS staying where I put it, so tend to use a strip of painter's tape to insure that item stays relatively put, even in practice stalls.
One other external device: I already have a stand-alone Verizon 3G "MiFi" unit. The MiFi is basically a small battery-powered Wifi access point <--> 3G network gateway. Makes it easy to get network access on the road for both my iPad and our laptops.
