Page Rank Sculpturing.

Webmasters tried to add nofollows to internal links to try and sculpt the flow of page rank, but google changed it so your page rank is divided across all your links, regardless of nofollow or not, so you will loose pagerank flow if you add nofollow to internal webpages.

http://www.bloggingtips.com/2007/12/05/increasing-pagerank-through-smart-internal-linking/