I have also experienced the same drop in PR on my inner pages.....about a month ago. My listings have stayed pretty much the same though.

The only thing I can figure is it has something to do with the way the pages are linked together. They have to be linked properly in order to "share" the PR to other pages.

The reason I figure this is because it all started the day after I added links back to my homepage from each of my product pages.

I have been going through removing each of the links I put in but it will take awhile as I have over 400 product pages.

Other than that I have no idea as to why this may be occuring...but it seems to be happening to alot of sites.