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
I notice this all the time. I don't think it's something "terribly wrong" in every case. It just means Google doesn't want to emphasize the page for some reason. Links pages have been losing visible TBPR with every update lately. The "thinner" their non-link content is, the easier it is to go gray bar.