Thank you! I was having the same issue (running from IDEA from IntelliJ - the best IDE on the market), and this took care of it. I was ommitting some of the requisit client libs from my classpath, however, and it resulted in the same error. I appreciate your foresight in posting the response to the correct forum.
The client side classes were under great movement, so we apologize for the fact that unmarshalling on old classes will throw exceptions. The classes are stabilized now but we will optimize the writing of it (work underway as we speak) as optimized writers bring about a 30% increase in speed. So you might see more of that in the future. I recommend posting this to the FAQ, with a note for Tobias to actually moderate it and let it through, as it will pop up again and again. Make sure to say "Client throws UnmarshallException after upgrade of JBoss".