Read Write Web has this great post on "Why Siri Matters". I'm a big believer that voice will be the killer interface for all things mobile. We are just in the very beginning of voice interfaces and I don't think we can even conceive the amount of change this will bring to our lives.
The most obvious implications are less reliance of desktop computing. What does that mean for companies like Google, Facebook, and Microsoft? With agents like Siri will most web searches go through Google? Will you do all your work processing with voice?