you would need to buy books from the US amazon site, and i think with a US registered CC…
I live in france, and have just bought a UK kindle (its awesome btw), using my UK CC, delivered to my dad – i am dowloading books from the UK site via a proxy server.
You could get an international one, which would have less restrictions, but there is less choice of books, they tend to be more pricey and the delivery times can be incredibly long.
this info is what i gathered from the kindle forums, and might be a little inaccurate, i am not sure exactly how the whole 3G thing works tbh – i don’t have this option.
have a look at the proper kindle forums, perhaps.