About once a month, I get one of these questions in stickymail. Something to the effect, if I click on my logo, the pr value changes.
In every case I've looked at, there is something of a redirect going on somewhere in the process. eg: you aren't look at the same pr for the same page.
Even though all of these generate the same page, they are different urls/pages:
long ip address to the site.
short obfuscated ip address to the site.
Google would see one as the real page, and see the rest as duplicate content.
Down into the site, the same thing can happen with directory names.
There are also transparent redirects, js loads, frames, and iframes, that can make the toolbar think you are at a different url even though you are staring at the same content in the browser view port.