I once tested an alpha of a flash based forum designed by a member of
kirupaForum - Powered by vBulletin (I can't remember who) and it was a pretty dull experience. It felt much like a regular PHP forum but slower.
There's only so many things you can do to a forum before it becomes user-unfriendly. Flash can't really add much without taking away from the accessibility side. PHP is the perfect language for it really.