During the latest backlink update on Google, my home page went from a PR4 to a PR0. SERPs did not change and my internal pages have PR3s, PR2s, etc. So what happened? Given my SERPs did not change, I would not care so much but it affects my coop ad weight a lot!!.
I first saw this happening with a competitor's home page after the June update. Since then, the same thing has been reported on a number of forums (and probably here too). I have yet to see a credible explanation other than "glitch". In the case of the competitor's page, it did regain its PR after the last update (or most of it anyway).
I also seen this happen to several sites, One site was PR7 index page went to PR0 with no change in inside pages. As I remeber it was temporary and got the PR they had.