A little over a week ago, I asked for an open discussion for dealing with the jerks who hack into sites to deface them, to install malware, or to surreptitiously insert hidden links to bad neighborhoods. (And if anyone wants to argue semantics of hacking vs. cracking again, take it up with Google this time - I'm just repeating the terminology they used).
Now, Google has written a long post giving advice on how to deal with this problem if it happens to you. They detail taking your site offline, assessing the damage, recovery, and restoring your online presence. Their post isn't the be-all, end-all answer that rids the world of this problem, but it's a great post to bookmark if you ever find yourself wondering what to do if you realize you've been attacked.