Haha, After 5 hours of working, I successfully migrate my Application from JBoss 2.4 + Tomcat 3.12 to 3.0.3+4.12 (the latest bundle).
My app uses JSP, Servlet, JMS, SessionBean, EntityBean, MessageDrivenBean, JavaBean and everything, and was based on SQL Server 7.0 through WebLogic JDBC Driver.
Now I succesfully migrate to not only the latest version of JBoss+Tomcat bundle, but also to SQL Server 2000 SP2. The only problem lies in the DataSource Configuration, Context Configuration.
Besides, Tomcat 4 obeys much more strictly to the specification, and helped me correct some little problems.
Thanks to all those answered the questions, I spent several hours reading and searching the answers, but, frankly speaking, JBoss documents has no clear outlines and steps for those unexperienced. So, I'll draft a more specific "Step by Step" article when free, maybe next month. :D