If you really want to try setting up JBoss clustering, but only have a single physical machine to play with, perhaps try some of the virtualisation tools that are available?
e.g. Qemu, VMWare, Parallels, Xen, User Mode Linux ...
They may take time to setup and they won't run "full speed" (so don't use them for performance testing) but it's a fun way to try out different configurations without having the actual hardware.