That's right.
Hibernate is proprietary from a single source.
JDO is a standard, gone through the
Java Community Process, it is supported on relational databases by 15 or so vendors, so far.
O'Reilly has another book called Java Database Best Practices. It does not go into much depth, but it covers
JDBC,
EJB CMP, JDO, Castor, Hibernate. It was written by someone who has authored a SQL and MySQL book, so he comes from a primarily relational world. He actually is quite positive on JDO, especially compared to EJB CMP. The sections on Castor and Hibernate did not go into much depth, and I have not looked at Castor or Hibernate directly, but I did not see HUGE differences with JDO, except that it is a different interface, which is proprietary and specific to those implementations. I can also say that I have talked with people that felt they really got burned by Castor's level of support. They have switched to JDO and are happy about that decision.