In ejbPostCreate() only the bean can pass a reference to itself ie its EJBObject and not in ejbCreate(). check what happens when u try creating an entity...u will understand the need for ejbPostCreate().
A very common use for ejbPostCreate() is setting relationships for CMR entity beans. You must do it in ejbPostCreate() snice it doesn't have a primary key yet in ejbCreate().
Don't get me started about those stupid light bulbs.