As a frequent International traveler who takes his Kindle everywhere, I have been asked several times if the Kindle can be used overseas. From personal experience, I can say that the Kindle has worked for me in South America and Japan. I have the Kindle DX model with the 6 inch screen, which is advertised as being usable in over 100 countries. I hope to be able to confirm functionality in all of the other countries listed in the future.
Using the Kindle overseas requires some planning. The user will want to make sure he has completed the registration process and linked a US credit card to his Amazon payment account. This will ensure that Amazon knows you are a US user.