Why wouldn't it?
Isn't Siri all handled server-side anyway?
If they upgraded Siri server-side, they would have to intentionally BLOCK older iOS versions from using it.
But then, that would mean they'd have to keep providing the iOS 5 version of Siri on a separate server to maintain it's functionality for the non-upgraders - which seems like a big waste of money on Apple's end.
However,, there's no Siri on the iPad 2, which uses essentially the same Hardware as the 4S, so I guess Apple is not above bully-marketing tactics with their own customers...
EDIT - I was just thinking that iOS6 may add new "strings" for Siri to access apps and other phone features that don't exist in iOS 5. I mean, you could probably get all the newer web-browsing features, like more specific sports-related web queries and the like, but as for app-launching and other iOS-based commands, you might actually need to upgrade to iOS 6 to get that newer functionality out of Siri.
Who knows..