Originally Posted by tworrell
Or it could be because of some other reasons we don't know about yet. There's some chatter that the mic might be different in the 4S. The point is that Apple does not have a track record of trying to cheat people out of OS feature upgrades. They usually have a good reason for not allowing a particular feature on certain hardware: either it won't work or it'll work like crap.
Originally Posted by redraider133
Another possibility is between redraider's theory and mine.
Apple will probably wait until after the end of the holiday quarter and bring Siri to the iPad 2 in the first quarter of 2012. Not only will this give market incentive for people to buy the iPhone 4S by having Siri as a device-exclusive feature during the holiday period, it'll also give Apple's servers and Siri's algorithms time to adjust and scale to the number of inquiries it'll receive.
Once Siri's beta period ends and international rollouts for the iPhone 4S are reasonably complete, that's the perfect time to bring Siri to the iPad 2 in a dot-update to iOS 5. By that time Siri and its supporting infrastructure will be more mature and able to handle the extra load of adding support for one more device type, and it should also help alleviate the typical post-holiday quarter sales slump. Source