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.