We were on 30,000 uniques a day slapped down to 12,000 - ouch.
We found some really dodgy links in WMT that we think killed us.
We build websites for a niche business sector and when we first kicked off the business in 2004 we backward linked from our customers websites back to our main website in a noscript tag. Not just one link though...about 70. Very dodgy I know, but as our business grew and the seo world evolved, we had our web designer backtrack the dodgy links and take them off all our customer websites as we realised that you just could'nt get away with that anymore. We have adopted a very clean seo method ever since.
The backtrack was in about 2007 - but guess what - our designer missed links from about 12 websites (from about 350 sites) and the pages that the links were on are dynamic so they have been multiplying ever since.
I found about 31,000 of these links in WMT all of which have been deleted on 27th April, but are still sat in WMT.
The WMT refresh could take months and I will update here when the links are gone, but it just goes to show that even some old sneaky stuff that worked years ago can come back to bite you in the a$$.
The confusing thing is that we were fine through Panda and have never had a links warning from Google.
I have been a bit of a watcher on forums since penquin not being able to talk about it due to deep depression and various bouts of kicking myself around the room.