Hackers have been trying to port Siri to older iDevices since the virtual assistant was announced alongside the iPhone 4S. Siri doesn't technically require the new handset's A5 processor and increased memory, so limiting the technology to the 4S is clearly a marketing maneuver.
We've already seen Siri running on an iPhone 4, so we know it's possible. But those instances required special circumstances, and we didn't know if we'll ever see a public release. Siri's dictation feature has now been made publicly available in Cydia for the iPhone 4, and it actually works.