Additionally in EJB 3.1 (JBoss AS 6), one can specify what needs to happen if multiple threads access the same client proxy simultaneously.
The default behavior in EJB 3 is to serialize access. In EJB 3.1 this is still the case for session beans other than a Singleton session bean, and obviously for non-asynchronous methods, but as an alternative you can now request that an exception is thrown instead.