FranticFish - 7:26 am on Apr 28, 2011 (gmt 0)
I was subject to an OOP once on an affiliate site due to a mix of low quality repetitive anchor text links and thin content that was over-optimised. The penalty kicked in when the site designer replaced a bunch of straplines (non-keyword headers) with images for the text. I guess the percentage of certain words together with the links was too high.
De-optimising the page worked. Then, as others have said, we got a bunch more links with other non-related terms in them.
I haven't worked with those sort of sites or links for some years now, but I've heard fairly recently from an ex-colleague who still does that Google still works the same way (N.B. this is pre-Panda).
If changes to the page weren't the straw that broke the camel's back, then de-optimising that might not help, but it's the first thing I'd try. If it's links that are the problem and they are low-quality, you might find that Google takes time to notice them - so you might have over-stepped the mark several months before you see the penalty, and you'll have to wait a similar length of time for Google to assimilate a new link graph and restore your rankings.