First off - I dont know why anyone would want to know what the PR (displayed anywhere!!!) of a page is (note I said page - and not site).
It keeps being bantered around as the bargaining chip for link trades etc and yet it is one of the smallest indicators of where a site will rank as far as Google is concerned. It certainly plays very little into how important Google thinks the site is since it is able to be manipulated so easily.
That said - the facts about PR are - the displayed PR on the toolbar is updated about every 3 months, the PR Google uses for its algorithm is updated almost every day and the best representation of a pages PR is in the Google directory listings - for example:
http://www.google.com/Top/Adult/Comp...Sites/Variety/
Note the pages have a little bar to the left of each listing - if you view the source of the page you will find the code used to make those imgs - /images/pos.gif" width=11, height=4 The width goes from 1-40 and it is somewhere around a mathematical logmarithic base 7 number, not linear.
Interesting stuff but it still has so little to do with where a page ranks in the SE results pages, that as Bill said - it might as well go in the "no practical use" bin.
I also have always had a standing rule that says if you contact me for a link/traffic trade - you better not mention PR or I automatically say NO.