I usually don't pay too much attention to PR as far whether it has a 2 or 4 or whatever. I do pay attention to whether a site has a graybar or not. I always assumed that old pages that had been previously indexed that got graybared where doing something terribly wrong.
In the last update I noticed a number of sites (most of which I don't own but know well) that had a high number of internal graybar pages. These were pages that I would have assumed would have a pr of 2 based on the other internal links hitting them. Most of these pages still rank in the serps.
I had a theory that it was related to duplicate content (category pages for a blong) but I found a number of cases where the content on the pages was totally unique to that page. While some pages were farther down the site structure I found some cases wehre a greybar page was one click away from pr 2 to pr 4 home pages