So if you are ever to market yourself as the definitive guru in any field it is a good idea to provide free supplemental data which solidifies your position. TextLinkBrokers just released a site which keeps track of what sites have PageRank blocked.

His blocked PageRank directory site is located at blockedpr.com

I am fairly certain that this bold move may make his firm a target for Google, but based on conversations I have had with him they do a good job of keeping their clients separated from their site, and they sell links smartly so as not to really raise warning flags. On his textlinkbrokers.com website he wrote an article with tips to not get your PageRank blocked if you are selling PageRank.