The problem is with org.jboss.mx.modelmbean.ModelMBeanInvoker. You can see the diffs here http://fisheye.jboss.com/viewrep/JBoss/jmx/src/main/org/jboss/mx/modelmbean/ModelMBeanInvoker.java?r1=22.214.171.124&r2=126.96.36.199.
However, this identifies the problem not the solution. I think you can request a fix of this version using JIRA.
So it's related to me using Java 5! And indeed, it works fine with JDK 1.4. That's a workable solution for now.