Since Siri operates from Apple's servers, I'm guessing that it won't be happening anytime soon, unfortunately. Although iPhone 4S has a good number of new upgrades/features, you can't ignore the fact that Siri is a huge selling point. If Siri's released to other devices, people will be more inclined to hold onto their current devices until the iPhone 5, cutting into Apple's profits for the 4Ss.
So yeah, unfortunately not anytime soon (at least via official channels, and I'm guessing JB channels may be difficult too consider the fact that it runs off Apple's servers)