Here's a theory that I have. From the changes I've observed (on one of my keywords). The number one has been up there for a while, and in the industry it means big bucks. Now that they are gone, all the money they made is going into PPC in this costly industry. This has happened to me before (2 times) and ofcourse I got into the costly ppc ads. IMHO this is business for G, help you make the money then "sandbox" you so you can, in return, stuff your income into their ppc system. G SERPS natural? Sandbox? I call it business! You make it, they take it.
I noticed a pretty big change this afternoon for about an hour or so and then my cache went back to dec 25...this morning it was dec 28....it was obviously an algo change b/c different snippets in the page were picked up that normally were never touched...it did go back to normal though reverting to this older cache...who knows!
I've seen some weird stuff going at google. Extreme bouncing up and down from #1 to #14 and all over the place. Also, listings that have little to nothing to do with the topic have crept up to be top listings. (Casino's, shopping sites, travel guides, hotel reservations all now in the top 10)
For example, I searched for "churches" and a site called Dungeon Majesty is near the top offering "Hot chicks playing D&D on TV!"
I'm seeing some serious strangeness... a couple junky sites with lots of outbound links with, I kid you not, PR ONE and TWO, are beating well established sites (PR 6 adn 7) in some of the keywords I monitor. Is Google experimenting with moving away from PR, on-page content, and, oh, I don't know... relevancy?