JBossAS-5 allows each EAR to have its own isolated classloader, by setting the isolated flag to true in the EARClassLoaderDeployer (%JBOSS_HOME%/server/default/deployers/ear-deployer-jboss-beans.xml):

<bean name="EARClassLoaderDeployer" class="org.jboss.deployment.EarClassLoaderDeployer">
<!-- A flag indicating if ear deployments should have their own scoped
class loader to isolate their classes from other deployments.
-->
<property name="isolated">true</property>
</bean>

However, this does not work. The classloaders for the EARs are not isolated. Please see the referenced forum thread for the complete details and the patch.