For any example that is deploying to a standard Java EE 5 environment (for instance the jpa example deploying to GlassFish or JBoss AS 5), the entity manager factory should be obtained through JNDI through a persistence unit reference.
5.3.1 of JPA spec:
"Within a Java EE environment, an entity manager factory may be injected using the Persistence-Unit annotation or obtained through JNDI lookup."
It would first need to be declared in web.xml:
Then the components.xml would look like this:
The reason we should do this is to advocate the Java EE standard approach.