This is complicated!
To BUY a nookColor you need to have a billing address in United States or in U.S. territories (American Samoa, Guam, Palau, Puerto Rico, and the U.S. Virgin Islands).
To BUY eBook content it's rather confusing. Here are the details from the B&N site:
Purchasing digital content from the Barnes & Noble eBookstore on your NOOKcolor: You can only purchase digital content if you have a billing address in the United States, U.S. territories, or Canada. You cannot purchase digital content if you have an international billing address. You can only purchase digital content when physically within the United States (including Alaska and Hawaii), American Samoa, Guam, Puerto Rico, Palau, the U.S. Virgin Islands, and Canada. When you travel outside of these areas, you can download digital content to your NOOKcolor that you have already purchased (if you have a Wi-Fi connection), but you cannot buy digital con- tent. Some items are not available for purchase in Canada.People have tried some "tricks" to get around these things with the nook 3G, Read more here.