Larrysart at SEOChat has a fascinating tale of ups and downs in Google's SERPs over the last several weeks for 2 of his sites. His suspicion is that the DigitalPoint Co-op created or excacerbated a sandboxing penalty and that when he created a landing page on another of his sites to attempt to get some traffic back, that site tanked as well.

The evidence is quite strong in this case and it begs the question - could Google be penalizing with such sophistication?