No, Siri is only available on the iPhone 4S.
However, if you would like a substitute that can do some of what Siri does (not as powerful as the 4S siri) then try to free app Vlingo.
If you don't like it, you can delete it.
For me, in its current form, I'd only use Siri to be able to text and drive at the same time... and Vlingo can handle this for me. It can also post twitter updates and facebook statuses, pull up Google Maps, and a few other things.