Todd Malicoat (aka Stuntdubl), had this to say about his post on the seedier practices of SEO:

The purpose of this post is also to demonstrate that you shouldn’t place the fate of your business in the hands of your natural search engine rankings alone. Be prepared to hedge your bets with alternative traffic sources if something like these were to ever happend to you. Don’t believe everything that you hear from a search engine rep.

I'll let you read the rest from him - he's got some good links there and a topic that's made for the press - destruction of business, sabotage, conflict, you name it...