You need to copy the JDBC driver to server/default/lib.
Use the ojdbc14.jar instead of the classes12.zip one.
Thanks for your suggestions, Ricardo.
I did mean server/default/lib rather than server/default/deploy/lib (sorry about that.)
Also, I have tried the ojdbc14.jar driver (with both JBoss 3.2.3 and JBoss 4.0.0) and had the same result.
Try to raise the logging level for "org.jboss.resource" and "org.jboss.ejb.plugins.cmp" to TRACE. Then check the server/default/log/server.log file for some clues.
Read the server/default/conf/log4j.xml file for examples.
Just to close this, I found out when I visited the client that the Oracle people had misled me - the username and password were not blank at all; they were wrong! There are two places where logon can be configured and the user had not checked one of them, which was overriding the JNDI settings.