I found a soltion to this problem, I upgraded my JBoss from 4.0.0 to 4.0.1.
I read in another thread (http://www.jboss.org/?module=bb&op=viewtopic&t=58228) that the only difference between the two version concerning the class loading is the default configuration. I tried to just edit my 4.0.0 configuration acording to the 4.0.1 Relese Notes and I got this to work, but I recieved some other exceptions at startup....so I decided to just upgrade my JBoss version.
Well, I have downgraded to 4.0.0 again, I got problems with my working web service client in 4.0.1 see thread: http://jboss.com/index.html?module=bb&op=viewtopic&t=59466
The TimedObject problem I had in 4.0.0 is solved by editing the configuration according to readme.html you find with the 4.0.1 release.