If you're using Spring 3.1.1 and JPA with Hibernate as your JPA provider, you're probably using Spring's LocalContainerEntityManagerFactoryBean with a persistenceProviderClass of org.hibernate.ejb.HibernatePersistence.
I was having trouble getting Hibernate's Session to get hooked up to Spring's JpaTransactionManager. To fix this, include the following entry in your Hibernate properties:
It's configured automatically when you're using Spring & pure Hibernate with either LocalSessionFactoryBean or AnnotationSessionFactoryBean, but it doesn't appear to be so when using LocalContainerEntityManagerFactoryBean.
Now, hopefully neither your nor I will forget again.