Webmasters are in a state of confusion over Google’s behaviours over last few months. Being de-indexed from Google is an unpleasant and stressful experience, but with the Florida and Jagger algorithm updates, the de-indexing was justified and could be corrected. The last algorithm update has not shown this consistency. Some of our clients and web-sites have been de-indexed from the Google search results for discernable reason, only to re-appear a few days later.
Google has admitted, however, that the problems may be due to lack of enough machines. Full-up Google choking on web spam? | The Register
More worrying, the machines are apparently full of spam and robot generated blog pages.