Based upon some of the comments, it looks like Google is going after interlinked sites that are under one WHOIS.
This could be an extension of the Google assault on blog networks on March 19.
The usual vague notices are also being posted in Webmaster Tools accounts.
With all of the assaults that Google has launched over the past year, their search results have not improved very much--if at all.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin