vordmeister - 7:20 pm on Mar 16, 2011 (gmt 0)
Spotted a massive drop for one of my sites starting on March 12th in webmaster tools. Looks like really frightening stuff, but I'm not the least bit worried.
I don't use Google Analytics. Instead I analyse my own log files every Sunday morning. I've seen the webmaster tools page do strange things before which have related in no way at all to actual traffic or actual Google referrals.
Just a thought. Are you sure you have the killer traffic drop? Or is that just what some third party that isn't working from your actual stats is telling you?