May be there is a bug in identifying the resource inside a Java EE container, but I would suggest you register the resource with GlassFish and switch to use persistence.xml with the JTA transaction type, and jta-data-source pointing to the registered resource. In that case, you won't need to use Persistence class to create EMF, but just inject it into the servlet.
HTH,
-marina
[Message sent by forum member 'mvatkina']
http://forums.java.net/jive/thread.jspa?messageID=480713