I'm having the exact same problem. I downloaded a stock jboss-3.2.3_nukes-1.0.0 and only edited the nukes-ds.xml file to access a local mysql server.
somewhere there is a copy of bcel.jar that is conflicting.
do you have any libs in your $JAVA_HOME/jre/lib/endorsed directory, or anywhere else on the classpath that jboss may be picking up?
this thread discussed the same topic: http://www.jboss.org/index.html?module=bb&op=viewtopic&t=46863
Thanks jae77. I was really stumped on this one. I blew away my system CLASSPATH; which only had Oracle specific libraries. I also removed the xalan.jar from my JAVA_HOME/jre/lib/endorsed directory. I'm now able to successfully deploy nukes.