netmeg - 1:52 pm on Apr 27, 2012 (gmt 0)
zarathustra2011 - there were two major updates in the past week; a Panda refresh on 4/19, and an anti-webspam (Brett calls it anti-seo) update that Google calls Penguin on 4/24. If you saw big drops on Wednesday, that's likely what hit you. You could spend some time in the Google SEO forum here reading up on it (but those items have drifted off into anti-Google rants and may not be helpful) or you could head over the searchengineland and look for anything marked Penguin; there's some good stuff over there. But it does sound like you were whacked.