The error is saying that all classes of the same package must be signed with the same certificate. Are you signing your JARs?
When you built your code, did you build it against 4.2.0 or 6?
Problem solved. The problem was due to an old activation.jar present in the war when it was being built against JBoss 4.2. Replaced it with the new jar from JBoss 6 and it works now. Thanks