in this topic http://www.jboss.com/index.html?module=bb&op=viewtopic&t=98478 the question of what is the proper (specification compliant) way to check if an entity has been persisted was raised.
And here http://opensource.atlassian.com/projects/hibernate/browse/HHH-2439 is stated that
checking non-null identifier value being available on an entity instance if its in persistent state
relying on questionable behavior
Ask on the JPA Hibernate forum.