Originally Posted by Peter (IMC)

In case you have a really big site of tens of thousands of pages (which is actually what Greg Boser is talking about, Webnauts) then you can significantly sculpt PR in the site. And I can imagine that in those specific cases it can have a positive effect on positions for some keywords. But I have to say that I suspect that there is also a negative effect on the number of keywords the site is found for which in turn means less traffic to the site.