wow this is a rather old post but I'll give it a shot... The PR can be invalid if the webmaster managed to set up a redirect from a high PR site witch acts as a supper donor Serf around the site arethere dramatic changes in PR across the domain? Check the whois info maybe you'll dig up some dirt there ( if the's a big media site acting as an owner see if there are backlinks to the suspicious domain)
You are welcome. It is a shame that we have to worry about fake PR, but at least there are discussions about it happening and people are becoming aware. Hopefully some people get saved from being scammed because of the threads about it that they come across which help inform them about the problem.
I use pagerank.net to check my sites PR and it will also tell you that your site have the valid PR or invalid.
Better and accurate is install Google toolbar in IE or FireFox, browse the page and then see the PR in the toolbar!!!!!