With all the guessing and theorizing about Google's recent PageRank update, there's now at least confirmation that the update happened, and why.
According to Loren Baker at Search Engine Journal, one of the affected sites, he received confirmation from Matt Cutts that the move was definitely targeting paid link sellers:
The partial update to visible PageRank that went out a few days ago was primarily regarding PageRank selling and the forward links of sites. So paid links that pass PageRank would affect our opinion of a site.
Going forward, I expect that Google will be looking at additional sites that appear to be buying or selling PageRank.
According to Andy Beard (whose site was also affected), another PageRank update is in progress.