To my best knowledge, there are too many aspects in human voice/speech that no machine is capable to recognize and understand.
Doing voice to text translation in a hand-held device was impossible 12 years ago yet it's available in most smart phones today. We're almost to the point of having a universal translator as seen on Star Trek when I was a kid, getting real close. Obviously it's not perfect, yet, may never be, but the approach Google took with sampling voice data for years on GOOG411 will probably be the same approach needed to finish the project - lots of samples.
All the technology is quickly coming together way beyond search, it'll be completely disruptive to what we know today. Combining all the technologies for search, speech recognition, visual recognition, etc. and then putting some basic intelligence like Siri behind it to make it all work autonomously to pull those technologies together to serve your needs quicker and faster, it's going to rock our world. Back to Siri vs Google,
I think the changes seen in what we consider 'search' and Google's role in mobile search is going to be much bigger than most expect. Look at mobile search, Google Maps and Navigation is used far more often as the dominant interface to Google Search as one would rightly expect it to be.
Using just what we have today, imagine just combining Google Search and Navigation, Google Goggles, Word Lens, Shazam, Yelp, Open Table, and a few other cool apps all under on personal assistant like Siri and it could be a real game changer but not a Google killer whatsoever, just a Google integrator. Google has wisely positioned itself as the dominant player with Google Maps/Navigation/Places so everything Siri needs to know Google already has and replicating that data is a next to nearly impossible task, nothing Apple is ever going to do with a few thousand servers and a copy of Nutch. The only real challenger to Google's position here is probably Bing at this point, not Siri.
Another game changer Apple probably hasn't anticipated will be that the ad revenue from visitors for the individual services Siri integrates will disappear as those services become transparent. This will either cause those services (Google, Bing, etc.) to charge a service like Siri to access their data or they'll simply block access by Siri and make the personal assistant essentially brain dead. If Apple were to implement it's own search back-end, it would be to protect Siri from this eventuality of being held hostage for excessive fees or being hobbled IMO. A little forward thinking and rambling...
Another potential model will be that the end user will pick the 3rd party services Siri uses and pay a monthly fee, such as new TVs offering direct access to Hulu, Netflix, etc. via the internet on top of the existing cable company VOD offerings.
The mashups will easily go beyond basic search, thinking Google is the only type of search is a bit limiting in what it means for mobile users.
Having something like Siri make a reservation at the nearest highly rated steak house with any open seating from 8-9pm and getting driving directions using a mashup of GPS, Google search, yelp, open table and navigation should be a snap, doing things like that in seconds that it might take you 10-30 minutes to manually do today. Now imagine the reservation mashup ties into movie times and Siri picks matching movie times and matches it with available Open Table dinner reservations and gives you a list of possibilities, and so on and so forth. In the future, your phone may be doing search all day long even when you don't ask it.
Big Brother won't be watching you but your HTC EVO 4G just might! Existing bleeding edge technology will allow phones to charge from ambient light and when power isn't a real issue, visual and audio sampling will probably be continuous and your phone might just start searching for things before you ask. Example, your phone might chime in "Do you like that new INXS song playing in the background? Would you like me to add it to your playlist?" or while driving "We're within 2 blocks of a Target, didn't you say you needed some socks and a new pair of pants the other day?".
Anything is possible as the hardware and mashups rapidly evolve and I think the smart money is that someone thinking along these lines will go into acquisition mode and snap up the best of breed and start working on the next gen apps ASAP and lock out the competition from using those services.
It's possible today actually ...
In the end Google, and Bing, etc., will just be one plug-in component of that mashup future unless they own it.
Oh wait, can you say Android?
There are some Siri-like apps for Android already.
Maybe Google already is working on one itself? :)