Sand - 6:33 pm on Sep 5, 2012 (gmt 0)
Did your rankings drop for all keywords, or just this one? If it's just this one, that doesn't match up with what I've seen in the past when a penalty hits. I'm not saying it's impossible, but it seems unlikely that only one page would suffer.
As for checking for dirty links, there are many tools out there that you can use to check out your link profile. But I'd start by examining which keywords / pages have fallen, and what you changed when you 'de-optimized.'