@TheMadScientist: I agree, and well put.
People tend to think of the SERPs as fixed results - or results that try to approach fixed - but I think of them more as an organic work in perpetual process, test mixed with result. And any illusion of fixed we had in the past was because Google wasn't updating quickly enough. But now there's Caffeine...
It looks to me like Google Search has been striving toward artificial intelligence and, in its ideal world, answers every query exactly as the searcher intends it answered, even if that means billions of different sets of search results per query term.
And Google's goal would also be to free itself from reliance on SEO as it gets better at doing what it wants to do.
After all, why would Google want to limit itself to showing widespread preference to certain pages, when the Internet is potentially unlimited? PageRank is a tool, but not the end goal. Google's goal isn't to fix anything in stone, but rather to know everything.