Siri now ported to custom iOS 4.3.5 on iPhone 3GS, iPhone 4(GSM), iPhone 4 CDMA (iOS 4.2.10), iPod 3g and 4g and Apple Tv 2g (iOS 4.3).
Apple's famous new 4S exclusive feature called Siri was succesfully ported to 3GS by iAmS1r1 dev team!
Test photos: Siri on 3GS - Photos
Based on one of geohot's exploits and ih8sn0w's research, team iAmS1r1 got the Siri feature ported on 3GS!
Against any expectations the app was NOT ported to iOS 5 but to iOS4 (4.3.5).The reason Siri is imposible to be ported on iOS5 for 3GS is that iOS5 has links to specific 4s components prohibiting porting Siri to other handsets.(as ih8sn0w suspected)
The other great issue standing between our 3GS and Siri was the fact that Siri connects to Apple servers, the connection first being checked if comes from a 4S.
We admit.It was a stressful and time consuming job but iAmS1r1 team managed to fake these servers and make the 3GS appear as an actual 4s.
Once we did this, the actual porting was easy.

source: Siri now ported to custom jaibroken iOS 4.3.5 on iPhone 3GS, iPhone 4(GSM), iPod 3g and 4g, Apple Tv 2g ( iOS 4.3 ) and iPhone 4 CDMA ( iOS 4.2.10 )