Kevin Kelleher wrote a piece named Yes, Google, Siri is a serious threat that is getting a lot of attention this week.
In short, the article says:
The question isn't whether Siri is a search engine that can replace Google's search box. It's a different kind of search -- one that could be the future.
I use Siri every now and then, mostly for doing emails and text messages when it isn't convenient to type them. I use it occasionally for "searching" for answers, but when I do, it is pretty darn good.
Siri isn't really a direct competitor, at least now, with Google. It is with Google Voice but not with Google itself. Siri may use Google for the answer. Siri currently uses Wolfram Alpha for answers, plus other vertical search engines and when that fails, it goes to Google - for now.
A WebmasterWorld thread has discussion around the topic. One person goes on to explain that people are done with ten blue links, they simply want the answer. I am not sure how true that is for all queries, for sure some queries, but not all queries.
We all should take notice of the word "one". I don't know whether we should take that literally, but if so, then the current Google organics, limited though they may be, looks like a vast smorgasbord. As I understand the word "one", it means only one and none other, which means if you are not THE answer, then you are nowhere.
The future of search is changing and changing fast - where will it end up will be very interesting.
Forum discussion at WebmasterWorld.