From documentation to pakaging tutorial :-)
The EJB3 specification requires that there be a .par archive that defines what a persistence unit is. A persistence unit is a set of classes which map to a particular database. JBoss EJB 3.0 allows you to put entity classes and mappings directly within a .ejb3 archive, which is not allowed by the specification.
Be warned - it looks that Beta1 can work with "old" EJB3 packages but specified dataset is ignored and default dataset is used !!!!
If you need your (non-default) dataset then use EAR format.
the spec requires .par files, but JBoss allows you to deploy them in .ejb3 files as well. I didn't want to totally break the older preview users.