Doug,
I came up with those same problems at Sun N Fun, which is a somewhat scaled down OSH in Florida where I have gotten a lot of pictures of aircraft that the registration didn't exist or that the plane didn't match at all - ie a Citation showing up when it was a Piper 32. I figured either FAA didn't catch up yet or that airport-data hadn't caught up yet and they were new regs.
Glenn was mentioning to me about fake regs too on aircraft in sort of an illegal action, but I don't know why anyone would fly an illegal plane to somewhere like Sun N Fun or OSH where there are likely several cameras pointed at these planes or a bunch of British guys writing down numbers in books
You brought up a good point though with the 30 day regs. I often ended up posting data on the site for what I could get from these planes
I was getting a little bit of that at the NBAA static displays too, maybe some of these regs were temp regs as these corporate jets would probably soon be bought by someone else as many were for sale.
Another thing that was interesting is the Dornier CD-2 Seastar used a fake registration while it was parked on display at NBAA. It's real German Reg is D-ICKS, however while it was parked on display it was displayed as D-IOKS - maybe for obvious reasons.
You can see it in these two pictures - there was only one CD-2 at NBAA
http://www.airport-data.com/aircraft/photo/244541.html
http://www.airport-data.com/aircraft/photo/236101.html