Siri was a free app for the iPhone before the 4S then Apple shut down those servers and pulled the app. Way better then AVI is Vlingo which was 6.99 and is now free.
There is also 'Speaktoit' as an iPhone 4 alternative, which offers similar functionality. I use it fairly regularly, though I've yet to try some of the cross-program features, like sending an email, etc.
Meanwhile, it offers a friendly, user-tweakable avatar - you can choose male or female - if that matters.