Recently I have been in the process of migrating my applications to JBoss 4.0 and doing the required testing. In that process I have detected a few problems with the 4.0.0 release, you can find the proposed patches on the following bugs reports:

• CMP beans always looks for the DefaultDS: This will be a problem only if you deploy a different datasource than DefaultDS, and you remove DefaultDS from deployment
• Problems deploying the same webservice from the same ejb-jar: happens if you deploy multiple instances of the same EAR with some J2EE 1.4 EJB Webservices

Update: Those bugs have been fixed and are expected to be release with JBoss 4.0.1