When you are running the application you may get an error like this:
This is because Hibernate used JPA 2.0 specification. But Google App Engine only support JPA 1.0 specification. I also got the same issue, and therefore I used Spring Hibernate support (instead of Hibernate JPA).
"The App Engine Java SDK includes an implementation of JPA 1.0 for the App Engine datastore"
Google App Engine JPA : http://code.google.com/appengine/docs/java/datastore/jpa/
Love to hear from you! feedback(comments) are always welcome