What version of JBoss are you using? We are doing this w/ 3.0.x and it works. (though we aren't setting URL_PKG_PREFIXES, but I don't think that matters).
That's JBoss 3.2.0beta2.
However, I've already found the problem. The machine where JBoss was running had it's hostname incorrectly setted up. It's server name (singapura.teleleste.int) was listed in /etc/hosts with an IP of 127.0.0.1. When I changed /etc/hosts so the hostname is associated with the correct IP, everything went fine.