Well I started a post and it went on and on and on.
(a bit like ariston washine machines) - the gist of it was that Google have made a load of changes, im still not convinced this is true.
I think they had a temp. problem which is why a load of sites got dumped. Its possible most of your pages got updated and it missed the page you are refering too via this 'unknown' problem. However this is my theory, I would wait for all qualified guru's to advise ;)
The Google index contains two types of pages--fully indexed and partially indexed pages. Your page is currently partially indexed, which means that although we know about your site, our robots have not read all the content on your page(s) in past crawls. This does not adversely affect your PageRank or your inclusion in our index. It does mean that we don't 'know' what to call your page, so it gets listed with the URL as the title and no description.
We appreciate the frustration this causes webmasters who work hard to make their sites accessible to users. We are working to increase the number of fully indexed pages in our search results to alleviate this problem.