phranque - 7:30 am on Oct 11, 2012 (gmt 0)
have you checked GWT or done a fetch as googlebot to make sure there aren't technical problems?
i would look at the updates that occurred immediately before your rankings drop.
panda 20 rolled out on thursday and was going to take 3-4 days; EMD (if that is relevant) rolled out the day before your drop.
Panda 20 - a Full Update - 2012-09-27:
Algo Change Targets Low Quality Exact Match Domains 9-28-2012: