Do you have any other versions of the RXTX library installed (e.g. in /System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Home/lib/ext)? Do you have a /var/lock or /var/spool/lock directory (you'll have to check using the Go > Folder menu in the Finder or from the Terminal)? If so, and there's anything in those directories, you might try deleting them. You could also try creating a uucp group using Netinfo Manager (in Applications > Utilities) and then rerunning the macosx_setup.command. Do you have any other software that access the serial ports (e.g. other physical computing tools or PDA sync apps or a firewall)?