Simply search the site information on Google. ( info:youtubeforums.com ). PR 10 does not belong to your site, it is actually Google's.
its clearly fake PR mate but you need to make the most of it while you can. Not by selling links unless you really want to and state whats happened, you'd be surprised the amount of people who would probably risk it... but, This is viral at its best! get an article written up quickly and submit to Digg, something like this could hit the front page and you'll start bringing in a shed load of traffic... and we all know what traffic = .... Good luck with it Jord
I know its only been a couple of days but I am supprised that Google has not done something about it already I thought they would have been fixed a problem like this very very quickly, thats assuming they even know about it I suppose.