updated 01:19 pm EDT, Mon June 4, 2012
Feature may be exclusive to third-generation iPad
Apple will finally bring Siri to the iPad with the advent of iOS 6, claims 9to5Mac. The site quotes "trusted sources," who suggest that the change will be one of iOS 6's main selling points. All of the features from the iPhone version of Siri are expected to carry over except for anything specifically phone-related. The iPad interface should also operate in the same way, the key difference being that iPad users will only see a small window slide up from below, rather than a fullscreen interface.
Apple has allegedly been testing the iPad version of Siri on both the third-generation iPad and the iPad 2. While the situation is uncertain, some of the 9to5 sources expect that Siri will be exclusive to the new iPad, much as it's currently limited to the 4S on the iPhone side. This may be to scale Siri support gradually, since any commands the software takes are actually processed on Apple servers.
Also unknown is whether Apple will show off Siri at WWDC 2012, which starts on June 11th, or if the company will wait until closer to iOS 6's launch, which may be in the fall if it coincides with a new iPhone. The sources do note, though, that Siri's voice recognition should work just fine on the iPad despite a person having their mouth further away from the microphone.