JBOSS provides moduler Java EE5 service in a customizable package, i.e. an EJB 3 container that can be part of an application running on command line, or in other words this container can be added in Tomcat to serve EJBs. So we do not need heavyweight AS anymore to play with EJBs.
Do you know if any such customized package(or embedded container) has been released from any other J2EE AS vendor?
OpenEJB comes to mind. It's definitely running in tomcat. If you limit yourself a little bit (no message driven beans) even the embeddable Container for EJB Lite could work for you. This one should be part of the JDK.