A Google Webmaster Help thread has discussion about a new linking industry, not for link building but for link removals. The concept is not new, we discussed it in 2008 with link unbuilding but it is much more of a concern today than in 2008.
In fact, so much so, there are companies cropping up that just work on linkage removal campaigns.
The webmaster pointed out two services, one deletebacklinks.com and the other linkdelete.com. SEOmoz is reportedly working on a way to detect spam links as well. Some other private tools do this already. There are also link removal tools such as removeem.com.