Siri works on the iPhone because the user always has Internet connectivity. Data plans are mandatory and can't be added or removed. I can't even recall the last time my iPhone didn't have data reception through either 3G or wifi. On the iPad however, the majority of users buy wifi only tablets, others buy the 3G, but then don't keep their accounts consistently active. Siri would only work over wifi in these instances. When Siri only works at home, it kind of breaks the illusion of being a personal assistant. In short, it creates a poor user experience, and I doubt Apple will incorporate Siri into iPads or iPods. I hope I'm wrong, I just don't see it.