Come on, how do expect anyone to help you if you don't say what jboss version you are using?
If you are using jboss 3 rc2 you do not have the entry in login-conf.xml set up properly to match the db-service.xml.
Thanks for the suggestion.
Turns out I'd added some .jar files to my jre\lib\ext directory that were hosing things up. I'm back on track now.
The url you gave was a great short description of how to get MySQL running as a datasource.
Thanks for your reply. Per my original posting, I was running jboss-3.0.0RC1. I've upgraded to rc2.
The problem wasn't with login-conf.xml (see my reply to lucluc).