Google confirmed that a Panda update was released on September 27th 2012, affecting 2.4% of English queries, which is an exceptional percentage for a Panda update impact. This update is not just a data refresh, but an actual algorithm update.
Google's official announcement from Matt Cutts:
"Google began rolling out a new update of Panda on Thursday, 9/27. This is actually a Panda algorithm update, not just a data update. A lot of the most-visible differences went live Thursday 9/27, but the full rollout is baking into our index and that process will continue for another 3-4 days or so. This update affects about 2.4% of English queries to a degree that a regular user might notice, with a smaller impact in other languages (0.5% in French and Spanish, for example).
This update was released at the same time the exact match domain update was released so if you're site was punished in this date you should also check for the possibility it was affected by the EMD update."