We had a customer at work who was trying to set up a DataSource in META-INF/context.xml as outlined here. He was using RHEL 5 w/ stock RHEL 5 tomcat / java (1.4!) RPMs. Unfortunately, no matter what he tried he wasn’t able to get it to work. After searching and searching we realized it was due to a missing file (naming-factory-dbcp.jar) in the RHEL tomcat install. Several others have also found this to be a problem:
https://bugzilla.redhat.com/show_bug.cgi?id=217141
https://bugzilla.redhat.com/show_bug.cgi?id=217630
We grabbed that file from the tomcat binary install tarball, dumped it into /var/lib/tomcat5/common/lib/, and that seemed to do the trick. However, we also had to upgrade java to 1.5/1.6 for whatever reason, so keep an eye on that also. 