P.S. Small anecdote about a site that got taken out in Panda 2.1 and came back yesterday. It's a small site, one of my earliest, and I hadn't set my spam traps properly, so it got over-run in the comments (each post with about 160 comments of spam). I zapped them all and expected the site to come back when re-crawled, but nothing happened - till yesterday when the re-gen happened.
Why did it need a panda re-gen to happen rather than a simple re-crawl? All I can think of is a) perhaps panda is looking for certain types of phrases it associates with spam, and once you are marked down it needs a panda re-gen to lift the blackmark or b) the links in the comments triggered the down-grade - i.e. the bot looked at the html and saw too many a tags, it tripped a ratio and down I went. And then got restored on the re-run.