JailbreakNation reports that a source close to Apple has tipped them off to news that Apple is testing Siri, the popular voice assistant receiving much media attention lately, on the iPhone 4. They claim that their source said Apple employees have been given access to a special software version that has Siri enabled on older devices - perhaps the iPad 2 and iPod Touch?
This news might not be very surprising considering that Siri has already been ported unofficially to the iPod Touch and iPhone 4, though it is still not widely available to the jailbreak community as of now. The major hurdle with enabling Siri unofficially is that all of the voice queries are sent through Apple's servers, which could easily reject any data sent from the wrong device. With an official release from Apple, a jailbreak workaround would obviously become unnecessary.
Siri has been a major focus of Apple's marketing for the new iPhone 4S model, so the company has incentive to keep it exclusive to their latest model. And their track record for enabling new features on older models is not good - MMS was never officially supported on the original iPhone and video recording, multi-tasking and Voice Over (the feature that Siri replaced) never came to the iPhone 3G. I tried to recall a case in which Apple subverted this trend, but drew a blank. Am I forgetting something obvious?