Spring 3.1 without persistence.xml gives “Unable to resolve persistence unit root URL”

I'm trying to use JPA EntityManagerFactory bootstrapping without persistence.xml, which is new in Spring 3.1 (since M2), but it appears that spring is still searching for a persistence.xml file, despite the fact that packagesToScan is specified.

In context.xml, the entityManagerFactory is configured thusly:

<bean id="my-persistence-unit"
    class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
    <property name="dataSource" ref="dataSource"/>
    <property name="persistenceProviderClass"
        value="org.hibernate.ejb.HibernatePersistence"/>
    <property name="jpaVendorAdapter">
        <bean class="org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter"/>
    </property>
    <property name="jpaProperties">
        <props>
            <prop key="hibernate.dialect">org.hibernate.dialect.PostgreSQLDialect</prop>
        </props>
    </property>
    <property name="packagesToScan">
        <list>
            <value>my.package.name</value>
        </list>
    </property>
</bean>

But on application startup, a PersistenceException with message "Unable to resolve persistence unit root URL" occurs:

...
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'my-persistence-unit' defined in class path resource [META-INF/context.xml]: 
...
Caused by: javax.persistence.PersistenceException: Unable to resolve persistence unit root URL
        at org.springframework.orm.jpa.persistenceunit.DefaultPersistenceUnitManager.determineDefaultPersistenceUnitRootUrl(DefaultPersistenceUnitManager.java:429)
        at org.springframework.orm.jpa.persistenceunit.DefaultPersistenceUnitManager.preparePersistenceUnitInfos(DefaultPersistenceUnitManager.java:327)
        at org.springframework.orm.jpa.persistenceunit.DefaultPersistenceUnitManager.afterPropertiesSet(DefaultPersistenceUnitManager.java:309)
        at org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean.createNativeEntityManagerFactory(LocalContainerEntityManagerFactoryBean.java:209)
        at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.afterPropertiesSet(AbstractEntityManagerFactoryBean.java:310)
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1504)
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1442)
        ... 34 more 
Caused by: java.io.FileNotFoundException: class path resource [] cannot be resolved to URL because it does not exist
        at org.springframework.core.io.ClassPathResource.getURL(ClassPathResource.java:179)
        at org.springframework.orm.jpa.persistenceunit.DefaultPersistenceUnitManager.determineDefaultPersistenceUnitRootUrl(DefaultPersistenceUnitManager.java:426)
        ... 40 more

Before I added packagesToScan, I had a minimal persistence.xml, and set the persistenceXmlLocation, like so:

<property name="persistenceXmlLocation"
    value="classpath:META-INF/persistence.xml"/>

which worked fine. So what am I doing incorrectly now?

edit added jpaVendorAdapter, error remains.

edit removed persistenceUnitName per a deleted comment; renamed beans to the persistence unit name (for @PersistenceContext)

update this error seems to depend upon which kind of application context loader is used. The code works in a spring/junit test via @ContextConfiguration, but not via ClassPathXmlApplicationContext