Apple Says Siri Won't Come to Older Devices

Apple replied to a bug report sent by Michael Steeber, founder of MSComputerVideos, and a contributor at Cult Of Mac. He had asked if it would be possible for Apple to add Siri functionality to older iOS devices, such as the iPhone 4, even for a price. Apple replied with this: 

Engineering has provided the following feedback regarding this issue: Siri only works on iPhone 4S and we currently have no plans to support older devices.

So, despite an earlier report that Siri could be coming to the iPhone 4, it turns out that Apple currently has no plans of doing this. Of course, it's still a possibility, but it's highly unlikely now.
via MacRumors and Michael Steeber's blog