Nothing, from the looks of it, google might be tossing PR updates altogether.
Google said in a video not to long ago that if they felt that "users" were not using google PR feature anymore, they would simply stop updating it "publicly" that is. Im assuming this was a hint that google know's the only people using PR are SEO'ers. If you think about it, both google chrome and IE can't use tool bars.
This will also take a hit on the Expired domain market since people mostly rely on PR when picking domains and if google hold's true as to what the were implying about not updating PR anymore, you'll never know whether your PR dropped or risen, of course google will still have this information.