Google has released some interesting tips on how to clean up a compromised website.  Websites can be compromised through a variety of factors including malicious code and conflicting information about site ownership.  If you are not on top of your game, your site can be de-indexed when this occurs, so it is important to use Google Analytics to keep on top of things.  In this article Google gives you a detailed explanation of the steps needed to put your site back in order.  It is a timely reminder to all of us to be constantly reviewing and fixing problems before they become catastrophic.