Duh. Colour me stupid.
I was thinking about it some more and had an amazing insight.
I was developing and testing on Sun Java Studio Creator with the Sun One Application Server, and I was happily updating the standardjbosscmp-jdbc.xml file in the JBoss environment.
Obviously, the Sun One App Server had no clue about the UUIDKeyGenerator at all.
I exported a war file to JBoss and tested it there ... it works beautifully. :)
Thanks for checking this post, guys. Sorry to waste your time. :)