I'm not working on any sites at the moment, so I can't tell you that I've experienced anything, but I doubt it's an issue relating to anything apart from your site. There haven't been any posts about it in sites I frequent anyway.
You say you check using the site: operator - this will only tell you how many pages are indexed on the specific data centre you are connected to - perhaps you've changed IP and you're now looking at a different data centre?
If you sign up to
Google Webmaster Central (especially the Webmaster Tools) you'll get a good picture of not only how many pages you have indexed, but many other valuable stats - incoming links for example are reported immeasurably better than with the link: operator.
Whether or a not a page is indexed is related largely to the page's PR, so are there any reasons why your pages might have lost PR? Have you lost any incoming links, changed internal link structure etc?
Google also has a habit of indexing lots of new pages quickly, then dropping the new pages with low PR - are the pages you've lost new pages or old established ones?
Finally, don't worry too much about this - if you can't find a reason for the pages being dropped then they'll probably be re-indexed reasonably quickly - Google likes to give webmasters a scare every now and again! If you find the problem persisting over several weeks then that's when you need to take some action.