It wouldn't be competing directly with Google though, would it? Siri offers a limited range of functions (currently, though this will inevitably widen as Apple makes more modules (and possibly opens it up to third parties)) whereas Google indexes (almost) everything on the web and searches by text.

I agree though: due to its nature of remotely processing input, Siri could become a search engine accessed via a web browser assuming the computer has a mic (which most current Macs — especially the biggest-sellers, MacBooks — do). It would become a single interface for accessing dozens of services — like Wolfram Alpha and Yelp — and tapping petabytes of information.