I seriously think that the idea about charging for a forum sucks. Indeed, you have a vibrant community around your boards, lots of great people around, but I don't think that all will stick around when the paid wall crosses their way into the forums, and even more do I believe that the growth will be nonexistent too.

I mean, you've got a great forum, but there's no "real value" that people should pay for using it. You need something catchy on it (something I can't think about right now) before you push through with this idea. It's especially a bit odd to imagine to have a paid forum for WORDPRESS which is a GPL software. It makes WordPress look more and more premium if you'll join the bandwagon of designers, authors and plugin authors "I am not earning and need to make money from WordPress Users".