Well, I actually watched the PR change on one of my sites yesterday. From 2 to 4 within 30 secs on refresh. So, let's see -- if I were Google and into making tons more cash quick, I'd develop a rotator for SERPS that is still reliable for the user delivering good results in highly competitive categories and thereby force more cash to the cows over at Adwords for those advertiser folks who must get back on page one. Makes sense to me.