I have two sites that have been hit by 30% today. Most terms have dropped 7 or 8 spots in the serps.
Both run adsense (not too agressive).
Both are authority sites from 1998.
One site has 325 deserved dmoz listings and numerous references from wikipedia.
Both have daily updated content.
Both are very useful and get / require a lot of attention.
If the cream eventually rises to the top, I'll be very relieved.
My theory is that google has devalued links such as dmoz, possibly implenting an "expiry date" for inbound links, and maybe taken away the extra points for having an "aged" website.
It also appears that it's only the US data centers that have been hit. I can see the original rankings on the .ca / .co.uk versions of google.