Google are phasing out PR and updates are happening less and less. It's averaging something like 8 months now instead of 3.
Larry Page (as in his name "Page" Rank) designed the page rank system while at Stanford University therefore they own the copyright to it not Google. Now Google had the exclusive 10 year rights to Page Rank but that's about to expire and everyone will be able to use it which is why it's slowly being phased out. They started this about three years ago when they removed it from the toolbar.
The only thing PageRank is good for is demanding better link exchanges from people who don't know or understand SEO that well.