This is currently the recommended way to access the javax.sql.Connection underlying an EntityManager:
OpenJPAEntityManager oem = OpenJPAPersistence.cast(em);
Connection con = (Connection) oem.getConnection();
It would be more convenient to simply write
Connection con = em.unwrap(Connection.class);
That way, the client needing to access the Connection only depends on the javax.persistence API and not on org.apache.openjpa.*.
I believe the JPA 2.0 spec neither mandates nor forbids this functionality. It is supported by Eclipselink 2.x.