A Transaction is started from an EJB as a Container Managed Transaction and then I call a DAO to execute some Hibernate code:

- is the Hibernate code implicitly part of the EJB transaction, or do you still have to call session.beginTransaction() within the Hibernate code to join the transaction already in progress?

If the EJB transaction rolls back, is the Hibernate code automatically rolled back, or do I have to detect this and then call rolback on the Hibernate transaction code?

This is done automatically. You may however include hibernate transaction handling code, to keep the code portable between application server and non application server.