That is how the Tomcat class loading works. Your web-inf/lib will override tomcat's loaded classes. So you can use different versions of xml parsers for tomcat itself, and separate ones for your app.
But the jboss version 3.0 uses the unified class loading architecture(See Scott's post in the documentation forum?? detailing this). In this case, Jboss should use the already loaded parser for the descriptors and not the one in WEB-INF/lib.
Any one else have any thoughts on this ?