See if this version matrix helps http://www.jboss.org/community/docs/DOC-10808
That does help indeed. I now realize I misunderstood the situation: it's not that versions -always- need to match exactly, but there are compatibility ranges.
I did notice though that the 2 incompatibilities I found (Jboss 4.2.2 client with Jboss 4.2.3 server, and Jboss 4.2.3 client with Jboss 5 server), are not listed in the matrix.
I think that the idea of having an API to check the versions is a good one. I recall many years ago while developing a client/server app that one of my testers complained almost daily that the build was broken. After a few times of helping him and finding out that in each case he mismatched the client and server components, I added a method to the server that returned the version number, and had the client request the version number before doing anything else, and generating a decent error message if it did not like what it saw. That tester never complained again, though he did mention that he often got that error but now at least he knew what he did wrong and how to fix it.