I also read the first post. I was able to locate the config.php file, locate the string value and change it, and then changed the admin folder name to refelect the change in value. I tested this and the acp opened fine.
Update: ...and everything worked fine until I tried to look at my forums while logged in on both bbs. I had to revert to the default and log in and log out of the boards separately, which suggests that the way cookies are handled may be a problem with two installs on the same domain.
Anyone else have this cookie problem? It manifested itself by letting me log in but returned me to the index page logged out.