tedster - 1:39 am on May 4, 2010 (gmt 0)
Knock sites up and down, measure user behaviour in different positions for different searches and gradually a more settled pattern will emerge. This can be achieved gradually with small jumps up and down - like continual A/B testing although it could take weeks, that's anyone's guess.
It also ties in with an infrastructure change which is likely required to collect and store the data, automate movement and measure the effects of each comparison.
Bingo - that's my suspicion, too. We saw a little bit of this behavior beginning two years ago, but it was rare and it seemed only to be used for URLs where something that helped them rank seemed a bit unnatural - even though it was not at spammy or outside the guidelines.
The bottom line for Google is not whether the website follows their guidelines. It's whether their useers like the search results.