There are a few thing that might go wrong, so I will tell you what helped in my case (analogous to yours, I was using JDK 1.6.0_13).
The problem lies in JARs mismatch. First of all make sure JBoss is using JDK 1.6 check your JAVA_HOME env. variable. Secondly make sure your classes are compiled using JDK 1.6. If you run your project from Eclipse make sure Eclipse is using right JDK, the same if you run JBoss from Eclipse.
I guess your have it right, so there comes some magic. Running client do not use libraries from $JBOSS_HOME/client, but use only the ones provided in $JBOSS_HOME/lib/endorsed. Obviously if you are using any other JAX-WS-unrelated API like JPA or EJB or Servlets you can add them to the classpath, but use only lib/endorsed JAX-WS JARs.
It helped in my particular situation in case of JBoss 5.1, in earlier versions of JBoss a few other tricks worked, for instance try to add java option
-Dsun.lang.ClassLoader.allowArraySyntax=true or/and -Djava.endorsed.dirs=$JBOSS_HOME/lib/endorsed when starting your client and JBoss.