If you need isolation of ur classes, you have to scope the classloader for each ear. Look at "EAR Scoping" in the getting started documentation.
You have to create a jboss-app.xml and place this tag <loader-repository>jboss.test:loader=urear.ear</loader-repository>
(Take a look at http://jboss.org/j2ee/dtd/jboss-app_3_0.dtd
This would create a classloader that only those classes in the ear use.
Thanks a lot for your fast answer! It works now.
Btw: this is a proprietary feature of JBoss, isn't it? Do you know if there is planned to support this option in upcoming J2EE specs?