Unless Apple has it patented up the wazoo. Which they no doubt do.
(As an aside; I think this is a completely fair use of patents. You're creative, you make a new patent, you get to exclusively use that technology for the next X years to build market share)
Which parts of Siri are so original that Apple deserves a monopoly on them for 20 years? Just take a second and think about how long 20 years is in software. Do you really think that no one else would have invented a similar product given 20 years?
Yes, patent lengths are rather absurd in this day and age. It should be on the order of 3 years. While it's easy for Apple to catapult new software into the market place, it can take several years for a small entrepreneur to market his technology.
The majority of Siri is the result of publicly funded academic research. Granted, that has not stopped the patent system from granting narrowly defined patents in the past (e.g. the famous pinch to zoom).
I doubt they've devised some fundamentally new algorithm. Like you said, it'll probably be some aspect of the UI. I think the Siri "way of doing things" is pretty radically new and I think Apple will be rewarded for that with some patent protection.
Do you have any evidence that they have patents which would preclude other players (especially those with existing similar products like google/android and Microsoft/WP7)?
Their UI is pretty radically new and it's reasonable to assume aspects of it are patented. I don't know of a competing product that works the same way.
(As an aside; I think this is a completely fair use of patents. You're creative, you make a new patent, you get to exclusively use that technology for the next X years to build market share)