My home page is a PR6 and it is showing as such in the toolbar. However all of internal pages are showing PR0 if I visit them directly by typing the URL into the address bar or clicking on the links in my site.
However if I click on a link to one of these pages in the google serps, the pages show up as a PR4.
I've seen different numbers twice. Once I went from a four to a three when I first started. When I looked at the cached site, I found a picture of my old site before I had upgraded, where the new site had been before. The next day, everything was back to normal. Somebody here at webmaster world told me that sometimes when they are doing some work on the system, they will display their old files.
The other time that it happened, was in the reverse. Sometimes my number was elevated to six, from a four, with my inside pages losing relevance to 0. Eventually it all caught up together. I was told that when google is getting ready to upgrade your number or downgrade it, that the various computers that google uses don't all do it together. In fact, I heard that if you watched all of the google computers, you can tell when an update is coming. I wonder if that might be your case?