The eVisibility guys, who gave out a super cool shirt at SMX West (see photo below), wrote an interesting post about the implications of the nofollow tag.
They use an illustration that describes how nofollow works by not passing juice onto the pages. It's actually an easy to understand diagram that explains how some juice passes to certain pages and others don't pass on to other pages, thereby allowing you to control the amount of link juice that flows to internal pages.
It's a really easy explanation with a simple-to-understand illustration that has been suggested by Sphinn members as "a must read if you are still scratching your head regarding the use of the nofollow tag internally."
Still, there are a few people who are perturbed by using the nofollow tool this way (using the Bruce Clay siloing example or the Rand Fishkin sculpting example, and there's a Sphinn discussion on the latter), but overall, if this is an approach that you still want to consider (for some non-critical pages, at least), the illustration makes it easy to understand.
Forum discussion continues at Sphinn.