greenleaves - 5:38 pm on Mar 29, 2012 (gmt 0)
There are now services for this type of thing. The funniest part is you can find them on google; just look up negative seo.
Having had a site sabotaged and seeing traffic drop by a factor of 80%, I can say this is very, very real.
Thanks Google! You can't distinguish spam to discount it, so now you allow legal, competitive sabotage. :S