If you have a Kindle with wifi (which I guess is all of them), then than will work all over the world. The thing I think is US-only is buying digital content, ebooks etc, on Amazon when you're not in the US. They have not said it is disabled when you leave the country, but that's my impression. I think it's based on media copyrights in different territories.
Same reason there's no reason for me to buy a Nook, can't use it outside the US, other than generic content from Gutenberg Project or whatever.
One time I did some research for a person living in a country where Amazon does not ship Kindles. If I were visiting that country, I could download books from Amazon.com as though I were home. I could even download using 3G (if that is available in that country) at no additional cost. If no 3G was available for my K3 then I would have to look for a wifi site. I presume that the Kindle Fire will work like that also. As long as I have a US account and mailing address, I can use the K3 all over the world.