Apologies, but the actual exception is:
Exception in thread "main" java.lang.NoClassDefFoundError: org/jboss/security/SecurityAssociation...
Make sure %JBOSS_DIST%\lib\ext\jboss.jar is in your classpath when executing your client. That's where the SecurityAssociation class resides.
Thanks phykell. I'm wondering if this little tidbit is documented anywhere? I hadn't seen it in my travels. The only JARs that were mentioned were the handful in the \client directory.