Tim Blommerde here on javalobby concludes his comments on JDO and Hibernate as
What I can say about Hibernate and JDO, but that's mostly based on what I heard at JavaPolis this year, where I heard Gaving King talk about Hibernate and Patrick Linskey talk about JDO, is that Hibernate seems to have become the defacto standard, where as JDO is more or less the theoratical and (semi-)official standard.
...Now I need to investigate whether it's true =)