gethan - 10:54 am on Apr 28, 2010 (gmt 0)
Ok - found something official from Google (Wednesday, April 28, 2010 | 2:11 AM): Known issue: Sitemaps indexed URL count showing zero indexed URLs [webmaster-forum-announcements.blogspot.com]
The indexed URL counts for Sitemaps that you have submitted in Webmaster Tools may be showing zero (0) indexed URLs for some sites. We are aware of this issue and are looking into it; you do not need to take any action. This will have no effect on your site's performance in search results.
Combine this with The Google site: operator seems broken - is this intentional? [webmasterworld.com] - and there are probably some very nervous webmasters at the moment... I'm trying my best not to be one of them ;)