We're upgrading from JBoss 3.2.3 to JBoss 4.0.2. We have lots of ears with wars and ejb jars, as well as some standalone wars, and a hibernate.sar.
We'd like to enable isolation. However, we've discovered a lot of classpath issues that were hidden by 3.2.3's non-compliance. It's often clear when one module is "reaching" into another module or ear when you remove the one it is undesirably obtaining the class from. What's baffling though is that once you do this the verifier doesn't always appear to be able to resolve classes in its module's own path!
In one case, we have a war that is finding a class in another ear. If you remove the ear from the deploy folder and restart JBoss, the war fails to deploy because it can't find the class, although the class is clearly in a jar in its WEB-INF/lib folder!
In another case an ear can't find a class in the ear's lib folder, even though it's correctly declared in the ejb module's manifest.mf.
In both of these cases, they find the class in another module, ear or sar if you deploy them first.