Dynamic Persistence Units in EJB3

UPDATE 10/27/08:

JPA FAIL! So, Mr. Patrick Linskey, who knows something about such things, points out that the standard actually does provide a framework for dynamic PUs: javax.persistence.Persistence.createEntityManagerFactory.  You still have to plug in vendor-specific properties, but this is a much better way to go.  I’m not actually sure how I could have been led so [...]