Well, two weeks ago my website Tolkien Library: bibliography, biography, resource, reviews, articles, essays on J.R.R. Tolkien books and collecting Tolkien was having a pageranking of 5. Then it dropped to 0 for about half a day and then ended up on 4. My site was having a PR 5 for over a year and so I suspect google decided to clean up or change some things that determing PR.
I do however think it does slightly affect the amount of visitors, but it could also be a coincidence...