Is Siri is new for every device with iOS 5, or just for the iPhone 4S?
Siri is only available on the iPhone 4S and only fully supported within the United States at the moment.

Unfortunately the page here on Siri does not make that extremely clear, apart from only mentioning the iPhone 4S. I can't find an official source on the Apple site other then that page that specifically says Siri is only available on the 4S.

Siri supports various languages, however some functionality, like local business searches, will only work in the United States at the moment.