Siri is only available on 4s. From what I have read on this site, users aren't all that impressed. I tried the Dragon app, which is like Vlingo, but it takes longer because you have to correct misunderstood words. Funny, suddenly we're all too busy to type.
It is difficult to get Siri on iphone 4 without switching to the 4s. According to Apple, Siri is only available on the 4S, for the time being. For it to work it requires a chip that needs to be fixed for it to operate.