jost, I have four sites and three have been hit. All unique content, no SEO games, I've already walked away from two sites that took years to build & have added 120 new (and good) articles, only to continue going backwards. In the meantime ehow & Yahoo answers outrank me. Ehow seem to be flavour of the month at the moment.
Who knows? They do what they do.
Having said that, my Adsense is up, despite traffic dropping. I removed two out of three of my link units & switched text/image on my 160 x 600 ad and my earnings have increased by about 30%.