Brucek
I have a Garmin Nuvi 310d with an SD card slot, I have bought a card from Garmin with the US maps on which I can then use in the States. It was about $130 in NYC.
Re: airborne use, I have used it on the flight deck of a 747 and it works fine, it shows all the roads and towns below and lakes and rivers, gives you an up to date position and a groundspeed. Obviously not for proper navigating but could be a help. You can also load custom POIs so I don't see why you couldn't load VORs, airfields etc. Battery lasts about 4 hours.
The max speed log looks pretty good, it's 580mph on mine, beat that Clarkson
If the CAA were really serious about avoiding airspace incursion they would supply suitable maps for free.