This happens randomly. My Rutgers site (one page, edu domain), was de-listed for a couple of weeks (didn't exist, even when I specified "site:rutgers.edu"), then just re-appeared. I also noticed this with Oliver Steele's functional.js page - I was used to typing "functional js" into Google to find it, and at one point, it disappeared for a week or two from ranking alltogether, only to mysterious re-appear again.

Maybe it's some clever way to check whether or not people track their SEO? De-list sites, and watch if the person reverts to an earlier version of the page, or submits it for Google to crawl?