we have exactly the same problem, with JBoss 3.0.6 (build with Jetty) on Mandrake 9.0.
waiting an ideea ......
somehow, finally it works. so:
i removed from java_home "ext" directory 2 jars (jbossall-client and log4j) which previously i have put, i made "build clean", "build" and now it works again. why? i really don't know.