We've employed JBoss 4.0 and the relatively new JCA 1.5 spec to deploy an Inbound resource adapter that listens to our device provisioning system (Cisco).
This has been a great experience and exactly what I have looking for - I feel J2EE 1.4 actually expands the horizons a great deal as to what Application Servers can do - especially with EJB 2.1 and JCA 1.5.
We are very pleased with JBoss 4.0.0!
RC stands for release candidate and is not the final stable version. The unqualified release is the final version.