I was just looking at this - http://www.att.com/ipad/#fbid=vHR6ydjkOHw
The details are not clear to me, so you would want to contact AT&T or whichever carrier you go with. It seems to me you are committing to a contract with AT&T no matter what - an installment agreement or a service agreement but the fine print indicates a service contract is required regardless (but I'd ask them for clarification). They also run a credit check (at least for the installment plan option), so nobody without a USA address and USA credit record or history will be approved anyway.
I'm assuming they use a SIM lock, as that is every carrier's standard method of safe-guarding their contract hold over you, but again, you'd need to hear that from AT&T to be sure.
I do see what you mean - if you go back into the "wireless" shopping options, and choose the "month by month" devices, you get the normal full retail price options.
All of these zero-down, upgrade whenever you want deals are fairly new with all the carriers, so chech them very carefully for all the details. And remember that you always could always upgrade with any carrier whenever you wanted if you were willing to pay full retail for your devices anyway - this is just a bit of flim-flam as far as I am concerned to make you think you are getting a great deal by putting little down initially, but you are on the hook for the full retail price in the long run anyway, just maybe with a service contract and restrictions on travel and use to go along with the deal.