The standard tomcat 5.5 tar ball from apache contains namingfactory-dbcp.jar in the CATALINA_HOME/common/libs directory while the ebuild for tomat-5.5.17-r1 does not set up a link to this jar. The jar was need while I was setting up DataSource Resource and Realm using mysql as the backend.
*** This bug has been marked as a duplicate of 144276 ***