My response might be a little late, but this is how I see it:
In the absence of any quality signals in the search landscape, the algorithm WON'T PENALIZE YOU for farmed links completely. I wish I had saved the test, but it's kind of like a blog comment, even for a nofo. If you have 4 different sites attempting to rank for a KW and the ONLY SIGNAL is a nofo comment - then that site with the comment wins. I COULD BE ABSOLUTELY WRONG, but I've tested this myself for the KW "pyoniz".
I suspect the same thing is happening with affiliate links for specific KWs with low quality search signals. In the absence of quality signals, Google will give microscopic weight to bad signals - and eventually it all adds up. FYI the bigger your name and the higher your PR the EASIER it is to engage in black hat SEO.
