What kind of XML configuration file? And what do you expect the deployer to do with it? For anything out of the ordinary (web.xml, struts-config.xml, jboss-web.xml, etc), you will need to write a custom deployer. If it's just something you need access to programmatically at runtime, use getResourceAsStream() to load it off the classpath (which should include web-inf/lib IIRC).
Actually it's not a concern now because the latest release of JBoss with Tomcat5 does unjar the XML file for me. The XML file is nothing special, just configuration info for my application. I do not want to use getResourceAsStream() because I want to be able to change the XML file and have the application hot-reconfigure itself without having to restart jboss. This is a simple thing to do but getResourceAsStream() does not allow for it.