netfleet - 12:43 am on Dec 23, 2010 (gmt 0)
Mate of mine has had something like this happen over the last few weeks but the site has been coming back then going again.
Homepage was ranking #1 for a high volume search term (801M results) then in a flash de-indexed. Then 2 days later right back to #1. Then yesterday (2 days later) completely gone again.
He does use some pretty aggressive and grey-hat SEO though.