A category page on our site disappeared during the last week of September. For about 3 years it was near the top of page 1 for a variety of competitive terms. No changes had been done to the page in about a year, except product additions.
I did a site: search and took a snippet of text from the page; we are no longer showing in the index. A product page from within the category is actually showing on page 2 for the same terms now. I checked Webmaster Tools and it doesn't show any problems and still shows it in the Sitemap. It still has Pagerank. It still ranks well in MSN and Yahoo.
Using CopyScape, I did find some scraper sites had copied a lot of the text from the page; they are actually showing up in the Google results when I copy a snippet of text. I went ahead and "100%-ly" changed the main content of the page.
Any ideas why this might be happening? We haven't built a lot of links directly to this page in quite a few months, but I would think it would have been penalized on a keyword level instead.