Yep I was able to use my unlocked 1.1.1 iphone to access the iTunes New Zealand store, buy an iTunes plus single, and sync it back to my computer.
I guess although apple only wants US iPhones, they are still not against making money thru other stores, otherwise they could of just limited the iPhone app to the US store.
Of course, it does have legitimite reasons, such as Americans with Credit Cards and Billing addresses in other countries, and with foreign iTunes accounts.