javax.jms.ObjectMessage is an interface. org.jboss.jms.message.ObjectMessageProxy implements that interface. Thus you can cast the received object to javax.jms.ObjectMessage without any problems.
By the way, if you ran your same code with another messaging system and looked at the type, you would find that the type is specific to that messaging system. And you will find that that type also implements javax.jms.ObjectMessage.