armondhammer - 8:40 pm on Mar 28, 2012 (gmt 0)
I think that the private-blog-network-mageddon that's underway has not only added fuel to this concept of negative SEO, but is starting to show more evidence of it.
I can say that I've seen some indication of a sort of algorithmic penalty associated with these junk links. It is worth noting that I'm in a space that is hyper-competitive and filled with dubious link profiles. Perhaps these are close enough to the edge to be effected, but there are definite major ranking shifts coincident with the recent PBN shakedown.