And if Apple succeeds with Siri's "one answer" search result, then SEO becomes a very strange thing to contemplate.
I don't think I'd worry about Siri as it exists today, it's an experiment, a toy. It's also a mobile thing and definitely not going to impact all aspects of search. I think eventually we're going to see a solid split in mobile vs. non-mobile search, that they'll become two different beasts because one size cannot fit all.
If technology like Siri is to become truly viable it won't be able to simply take the first result and assume it's the correct result. The preferences of the individual will need to be taken into account plus crowd sourcing the preferred results as the most popular result isn't always the first result shown.
When you truly have crowd sourced search results trends will emerge that certain types of people with certain interests will pick certain results and I believe search will be specifically tailored to those groups. Whether or not you know which group you're in or how to address a group via SEO will be the real trick in the future.
I have a sneaking suspicion Google could already do this today if they wanted to or are real close to being able to do it. When it happens it'll be a completely disruptive event to SEO as we know it. At that point SEO, if it's still even possible to do, will be so complicated most will probably throw in the towel and go social all the way.