I love Siri. You love Siri. We all love Siri -- and we want it on our iPhones 4 and 3GS, not just on the 4S. Numerous iOS hobbyists including limneos, Gojohnnyboi, and iH8sn0w have been hard at work trying to bring Siri over from the 4S to other platforms.
After weeks of work -- this effort predates the iOS 5 release and the 4S debut -- they have made substantial progress. What you're looking at here represents some preliminary results, enabling the assistant in the device settings.
The Siri-porting team is still working on launching the background daemon, providing the interactive magic dialog "pocket" and other system features.
Can you expect Siri on your jailbroken iOS device any time soon? I'd say you're probably looking at weeks rather than months or days at this point, and that is contingent on the team's ability to pull together many working parts including frameworks, daemons, Mobile Substrate overrides, and more.