when you have a US-states-based forum, with a sub-forum for each state on each of your hundred forums, you can easily split your count to 50 times less forum by using thread-prefixes instead of sub-forums... (each state is a prefix)

using prefixes as identifiant for topics will cut down the need to have forums as identifiants... you have the best example here on vb.org... they use prefixes to identify each type of hack release here, so instead of having 15 sub-forums for each vb version, there is 15 prefixes for each version... and you can choose to display only prefix X or Y, so it helps making more seperation on topics...

the worst way to manage big forums is to split the content by forums...