A user on WebmasterWorld mentions that his page rankings have dropped considerably after what appears to be a hack attempt on his server. According to the user, a vulnerability on his web host was exploited, and 50,000 new pages on a subdomain he never created (about prescription drugs) were being crawled by Google.
A similar issue occurred in October when Barry found that Jennifer Convertibles was delisted on Google due to a similar explioit.
After Barry submitted a reinclusion request on behalf of Jennifer Convertibles, the page began ranking again. Similarly, the WebmasterWorld user has acknowledged that he has submitted a reinclusion request and his pages are being crawled more frequently.
So far, the bot has been crawling a few more pages each night, now covering about half, up from 2% two weeks ago.
The user did say that he recently registered his site on Google Webmaster Central, though it may have been helpful for him to have done so earlier so that he could have recognized that his site was not meeting the webmaster guidelines.
Forum discussion continues at WebmasterWorld.