I don't know how much of Siri was Steve. In the biog, it's implied that he was given a demo of Siri for the first time at a Board meeting just before he resigned and was at first pessimistic that it could do as claimed. He asked it several questions and was apparently impressed.
If Steve was not heavily involved with Siri, that actually is a positive for Apple. There's an impression that Jobs was a micro-manager who was involved in every design decision and there's a more positive outlook for Apple's future if a wider variety of executives and managers were directly involved in everything we love about Apple's products that frequently gets credited to Jobs.
There is one thing about Siri that bothers me. If it's really only a Beta, then IMO it shouldn't be the focus of Apple's advertising for the phone. Apple wants to have it both ways: if it works, it's a reason to buy the phone and if it doesn't, "well sorry, it's only a Beta."