We've successfully installed WPMU here at http://bicyclejournals.net.
The owner of the space decided to pay someone to install an Invision Powerboard forum on the same domain.
It's address is http://bicyclejournals.net/forum.
He submitted a ticket to our hosting company's tach support. This is the reply.
"You will need to modify the rewrite rules since you are using wordpress,
that will keep the forums url from showing since the main .htaccess file
will apply to subfolders also."
Any advice?