If you are looking at a 3G sim card one, they are locked to the provider, AT&T or Verizon, so another reason not to buy it.
You are correct about Verizon as they don't actually have a sim card, it's built in. AT&T Ipad's are not locked to that sim card. I have purchased 3 of them from the USA, no problems with any.
Jayteeto..... you have to ask yourself if its worth buying without a 3G connection. When you get back home you can get a PAYG sim card.
Mutt