This comes up as a hypothetical risk whenever stories like this break, but I know of no cases where it happens. That includes situations where individual link-buyers and link-buying sites have been publicly outed by commentators or competitors.

The economics of this only work out well if you have high marketshare (if you're 5% of the market, and you blow up a competitor, 95% of the benefit accrues to the other 95% of the market). So I think this threat is overblown.

Every time it comes up, I've asked for examples--if you hear of any after the fact, I would greatly appreciate it if you'd get in touch. Email's in the profile.