At Windows Phone 7 Backstage (registration required), the international carriers for WP7 have been announced, though it’s a scanty list to begin with, mostly in western Europe with one in Asia and one in Australia. You’ll be able to use your WP7 phone with:
India, China, all of South and Central America, Africa and pretty much everywhere else? Sorry folks, you’re out of luck. Hopefully the platform will head to more countries in the near future.