Just as these guys have said, Apple and AT&T are both requiring in-store activation negating the possibility (though it's possible via an Early Termination Fee) of buying it and then unlocking and reselling the iPhone. That's Statside at least, I know that they allow for online purchasing overseas.
For reals! I actually have considered selling my 3G on eBay and then using the proceeds to buy another at the unsubsidized price. At $500 assuming I get $800 or so for the 3G I'd be making out pretty well. I wouldn't have to cancel my contract either, which makes that even better.