chrism - 8:00 am on Jun 7, 2011 (gmt 0)
I had a site which dropped by an average of 6 positions for its main search terms around Panda 2.1. I didnt notice any effect at all in the fist wave (or second wave I should say as it's a UK focussed site and I monitor the .co.uk SERPS - I believe the first wave was US only but may be wrong).
Having repeated a process a couple of times I've now successfully restored original rankings twice, after the first time reversing the on page change and rankings dropped back an average of 6 positions again. By repeating the change, once again the site's original rankings returned.
Each time the change took between 3 and 5 days to be reflected in the SERPS, even though we get crawled each day. Could this back up the Matt Cutts quote that Panda calculations don't run daily, but perhaps it do run every other day or perhaps third day?
The change I made was simply to move an Adsense block from the middle of content, between paragraphs to a floating, right-aligned div at the top of the article. Ironically its actually more in view in the new position as the page loads.
I can think of two possible intentions for this triggering a reranking, firstly that users perhaps see an ad with text wrapped as less intrusive than breaking up an article, or perhaps the problem is ads which are partially above the fold.
I fully accept that Panda will be more complex than just this variable, however I'm certainly looking at this as a tipping factor in my case that invokes the Panda rerank.