Digging deeper into the background, Siri was available as an app, not as fully-integrated with all phone features, but available for the iPhones 3 and 4, but that's now been pulled and the servers will be taken down on October 15th.
http://techcrunch.com/2011/10/04/the...ervers-killed/
I don't know whether the improved Siri requires hardware features not available on the iPhone 4, but taking the servers down for existing users of the older application does look a bit mean and simply intended to drive users to the 4S.
Graham