There is SOME hope that the iPad 2 might get it eventually, but even that is a stretch.
As Apple's servers have struggled to keep up with Siri usage during this launch, I've come to realize the huge investment in the server back-end that Apple must have made, and is still making, as the iPhone 4S rolls out and they tweak and improve the system.
That's not something they are going to just give away for free, particularly since enabling Siri on older devices would suddenly introduce millions upon millions of new Siri users to the already overtaxed servers. Those who bought earlier generation devices are getting all the other features of iOS 5 and that's a pretty sweet deal if you ask me.
One of the reasons I paid to upgrade was to get this specific feature. I'm fine if they hand it out to the masses, but only if their servers don't blow up. Otherwise I'd be kinda unhappy about it.