rango - 12:21 am on Aug 23, 2012 (gmt 0)
@hitchhiker, considering the dates and the quick drop I'm afraid Panda really is the most likely explanation. In which case, welcome to hell :(
My personal advice - first do a technical SEO once-through; ensure you don't have pages doubled up in the index, etc. Then, all that being in order, focus like hell on finding ways of turning as many as possible of your New Visitors into Returning Visitors.
There's lots and lots of reading on Panda out there; many of it conflicting; much of it pure speculation. My main take-away is to focus on user-experience as much as possible rather than the traditional SEO stuff like title tags, keyword density, etc. Plus, if you go tweaking your title tags too much in response to this, you could find yourself in even deeper trouble (as per that referred thread).