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From what I gather from the keynote, Siri is an iPhone 4S exclusive. I wouldn't be surprised to see a iPad 2 port via jailbreak somewhere down the line though.
 
small print on the iphone4s email from apple



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Siri is available only on iPhone 4S and requires Internet access. Siri may not be available in all languages or in all areas, and features may vary by area. Cellular data charges may apply.
 
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