I've got the exact same problem.
Did you solve your problem in the mean time? anf if so, how?
I had the same problem. SOmehow, I managed to solve it. It's a JBoss issue, concerning de XPath class from dom4j. In jboss >= 4.0.3 there is a custom package of dom4j, which should be replaced by the version which has the XPath class. I have copied the dom4j-1.6.1 and the jaxen jars into the lib folder of jboss and it worked just fine. Strangely enough, I had to copy the 2 jars as dom4j.jar and jaxen.jar (the dom4j-1.6.1.jar name did not work). After that, I kept on aop-ing.
I just did the same thing - copying dom4j-1.6.1.jar and jaxen-all.jar to JBoss-4.0.3SP1 /lib, but it did not work. Has anyone else been able to resolve this issue successfully?
Works for me (I add dom4j and jaxen in /lib and now I have XPATH). Is this in just a WAR or a WAR within an EAR? Regardless, JBoss should check your dom4j in WEB-INF/lib before loading the cached boostrap one in the UCL.