When java:comp/EJBContext is looked up in a method marked @PostConstruct of a bean, a NullPointerException is thrown from EJBContextFactory:

Caused by: java.lang.NullPointerException
at org.jboss.ejb3.EJBContextFactory.getObjectInstance(EJBContextFactory.java:57)
at javax.naming.spi.NamingManager.getObjectInstance(NamingManager.java:304)
at org.jnp.interfaces.NamingContext.getObjectInstance(NamingContext.java:1483)
at org.jnp.interfaces.NamingContext.getObjectInstanceWrapFailure(NamingContext.java:1500)
... 70 more

Please see the referenced forum thread for complete details. I was able to reproduce this issue with a SFSB too, so it's not specific to MDBs.