Seems like you're mixing the wrong jars/jre's in your classpath. Probably IBM adminclient jars and sun JRE? Or wrong xml/soap jars?
I never actually tried to call the adminclient jars directly. A better supported way is to use IBM's wsadmin.sh script and call the AdminClient interface via jython. Behind the screens, this will also do what you did from java, so you could reverse engineer that script to see which jre and which jars are actually used to make the magic work.
See http://pic.dhe.ibm.com/infocenter/wasinfo/v6r1/topic/com.ibm.websphere.base.doc/info/aes/ae/rxml_commandline.html for websphere v6.1 documentation on wsadmin.