Many of us have heard rumors about Google banning IP addresses or even entire blocks of IPs based on spamming or manipulative activity coming from them. Here's a case of an SEO whose found exactly this to be the case. From the post:

1. Google will not cache the index pages for any of the sites
2. Incomming traffic from google has dropped for ALL sites on the server
3. Google sitemaps will not accept sitemaps from any site on the server

I do wonder about the level of bad activity neccessary to spur on such an overt move from Google...