Siri is Apple's secret weapon. It will change computer interaction in ways we can't yet predict. I hope that when Siri comes out of beta it will be made accessible to iPad2, iPhone 4 and iPod Touch users on iOS5. That would dramatically expand the usage of Siri and prompt developers to integrate Siri in to their applications.
Imagine an iOS application that uses Siri to control your TV, your desktop computer and other devices.