Sand - 11:28 am on Sep 6, 2012 (gmt 0)

It actually doesn't seem to be the page. The keyword is "widgets". My second highest-traffic keyword is "purple widgets" and I'm currently #1 for that keyword. Both keywords relate to my widgets.html page.

To me, this doesn't sound at all like a penalty. Rather, it sounds like Google simply doesn't think your page is as relevant for 'widgets' as it used to be. There could be many different reasons for that, but the de-optimization is likely the culprit IMO. If it's possible for you to compare the current version of the page with a version from a few months ago, that might set off some lightbulbs.