Alternatives to HibernateUpdate: See my wiki page here for a more recent overview.
I just came across Carbonado, another persistence abstraction for Java. This is another alternative to Hibernate/JPA.
Carbonado implements the ActiveRecord pattern, which pursuits quite another design-paradigm than the DataMapper pattern, which usually is implemented by JPA and Hibernate.
As a reference I try to list the current persistence frameworks for Java. The list is certainly not complete... please leave a comment if you know another framework.
Most familiar are the mainstream JPA providers :