welcome to the difference between advertising and reality
we've got the same problem: about 1.000 sub-forums, still growing (we are doing a music magazine and want to build a sub-forum for each artist portayed on our site, that makes about 3.500 sub-forums) and that doesn't seem to be possible with vBulletin.
does anybody know a solution to this or do we really have to look for a different forum-system (which would be a great pity)