aristotle - 2:29 pm on Jul 26, 2010 (gmt 0)
As a blog owner adds more posts over time, so that more archived pages are created, the PR to each individual achived page is gradually diluted. Possibly this could be counter-acted if the blog is popular and continues to attract new backlinks. Otherwise, backlinks from an archived page will decline in value.
Something similar also happens with forums, as new threads are created, so that the PR transferred to old threads is gradually diluted.