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Although there hasn't been an official announcement from Amazon, the store is now also available in Europe, India and Australia. Until now, users from countries outside of the US could use the Appstore, but they had to register with the U.S. version of Amazon; now, they can use their localized Amazon account.
However, the store hasn't opened its doors for everyone - some users from the aforementioned countries report that they still can't access it.
If the Appstore is available to you, it's well worth a visit. Beside offering thousands of Android apps, the Appstore features a free daily download, and you may find it better organized than Google's original Android Market.