Apple isn’t planning on supporting Siri on older devices
We’ve just posted how Siri is successfully ported on iPod touch and iPhone 4 that ultimately clears the speculation that Siri would only run on iPhone 4S’s faster processor. However, there’s no official hope that it would be available for older devices, as Apple have made it clear during the launch of iPhone 4S. Basically, Apple has no plans to support Siri on older devices.
On a report of MacRumors, Michael Steeber posts an Apple reply to a bug report which states quite clearly that Apple isn’t planning on supporting Siri on older devices:
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.
From here we can just keep our fingers crossed until Apple revealed the announcement of official Siri support on the iPhone 4 or iPad 2.