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.
No you're right, they are not ready right now, but they seem to be heading toward that collision somewhere down the road.
I was thinking mic would be necessary too but they would certainly have to have a fallback for machine's that lack that capability (aka, a text input box box). Apple could even do their spin on it and have the question spoken back to you after you enter it so Siri can 'hear' it (Apple has to spin it unique in some fashion similar to how Bing has a background picture distinguishing itself).
For some reason I didn't even consider that you could control it via normal text input, but you're obviously right as that's what Siri will eventually use after the voice-recognition software is used. This totally widens the possible market then, you're right.
I think the main bottleneck at the moment though (and why Siri was limited to the iPhone 4S even though the 4 and 3GS run it perfectly) is due to server load. They simply don't have the infrastructure to handle all the requests.
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.