Matt Cutts recently announced about “Google’s PageRank Flow”:
So what happens when you have a page with “ten PageRank points” and ten outgoing links, and five of those links are nofollowed? Originally, the five links without nofollow would have flowed two points of PageRank each (Nofollowed links didn’t count toward the denominator when dividing PageRank by the outdegree of the page). Google has now changed how the PageRank flows so that the five links without nofollow would flow one point of PageRank each.
Nofollow links still don’t pass PageRank and still don’t pass anchortext so they don’t help sites rank higher in Google’s search results. But nofollow links now take away PageRank juice from your blog. Note: By default all the links in comments section in WordPress are nofollow.
The more links you have on a page, the less PageRank value each link will get. The more links (both follow or Nofollow) you have in comments, the less PR value is passed onto your normal, non-nofollow links in your blog navigation, blog sidebar and your blog content. [via HowToMakeMyBlog]