As posted in this mail  by Ed Keen there is a issue in the ActiveMQEndpointWorker.
I checked the spec the JBoss guys mentioned and in section 10.3.2 it is clearly stated:
"Both the run and release methods in the Work implementation may contain
synchronization blocks but they must not be declared as synchronized methods."
Removing the "synchronized" worked for some people trying this. But I think it was added intentionally, so somebody should check an appropriate alternative.
This bug prevents ActiveMQ from being deployed on JBoss AS 7.1