From my mail to dev list:
--------------------------------------------------
...
It's about "placeful service" and packages that want to contribute
@Entity beans to that service.

Currently we already have "notification service" contributing bean to
placeful service, which is done in deployment-fragment.xml of
notification service:
...
<extension target="persistence#CLASS">
  <class>org.nuxeo.ecm.platform.ec.notification.UserSubscription</class>
</extension>
...

Above will become part of persistence.xml file inside
nxplacefulservice-facade.jar directory in nuxeo.ear on jboss startup:

<persistence>
  <persistence-unit name="nxplacefulservice">
    <jta-data-source>java:/DefaultDS</jta-data-source>
    <class>
      org.nuxeo.ecm.platform.ec.notification.UserSubscription
    </class>

    <properties>
      <property name="hibernate.hbm2ddl.auto" value="update"/>
      <property name="jboss.entity.manager.jndi.name"
                value="java:/nxplacefulservice"/>
    </properties>
  </persistence-unit>
</persistence>

So far so good.

But the problem is that on jboss startup Hibernate persistence provider
will do scan of jars to find @Entity beans defined in persistence.xml
file and in our case if notification service is not yet deployed
provider fails to find that @Entity bean.

This leads to the fact that we need explicitely modify
nxplacefulservice-facade's deployment-fragment.xml to include required
jar containing @Entity bean(s) so that it is deployed before Hibernate
analyzes persitence.xml file in placeful service:
...
<require>nxnotificationservice.jar</require>
...

which sucks, but actually does its job.

The implications are that at the moment every package contributing
@Entity beans to placeful service will need also to modify placeful
service's deployment-fragment.xml to include itself as required to allow
Hibernate provider to find @Entity beans.