HenryUK - 4:33 pm on Dec 9, 2010 (gmt 0)
I don't suppose that I'm adding very much intelligence to this thread by saying "me too", but I'm always reassured when other people do this with a problem. So, me too.
My sitemaps (607 of them) all have loc, lastmod, changefreq and priority specified. Since early today the reported URLs in index have been dropping to 0 on them, one by one.
This is happening irrespective of page type, and plenty of URLs that are in the affected sitemaps are still showing up in Google SERPs.
Feels like a GWT bug to me. Hope so.