Add $jboss/client/lib/jboss-common-client.jar to the classpath of your client.
I was missing the jboss-common-client.jar file in my classpath. I now have jnp-client.jar, jnet.jar, jboss-common-client.jar and jbossmq-client.jar in my classpath and I'm still getting the same error. I'm running version 1.3.1_05 of the JDK on the client side (due to problems with the debugger in JDK 1.4.x under Windows XP). The server is running JDK 1.4.1 on Windows 2000.
Nevermind... I completely rebuilt my project (Sun One Studio) and classpath and things appear to be working now! I think there was an older version of Log4j in the classpath ahead of the newer version. Thanks for the pointer!