It's interesting how the nofollow tag has evolved over the years. Originally, people used it not only to control blog SPAM but also to shape the way PageRank flowed throughout a site - a method called PageRank Sculpting. The idea was you'd nofollow links to unimportant pages on your own site so that more PageRank would flow to the important pages you wanted to rank for.

I'm pretty sure Google has confirmed this doesn't work anymore. Now a nofollow link still takes a portion of a page's PageRank just without passing it along. Hence, no more sculpting power.