Although I haven't heard this said anywhere, the more I think about it, the more it seems to me that there is a lot in common between message based systems, also known as Message-Oriented-Middleware (MOM), (such as the Java Message System) and web services. Web services are really nothing more than a bunch of text messages sent back and forth among web sites. A message based system is designed to do exactly that — and more.
So my question is: why hasn't anyone developed a MOM service as a second tier on top of the web? I can understand why people might not want to commit themselves to it. (They would be captive; it may not be reliable; etc.) I can also understand why it might be hard to make money from it. (There is no advertising. The messages are read by programs, not people. So you would have to charge for the service.) But even so, it seems like such a potentially useful thing that one might imaging an open source system being developed. Why not? Also, such a system would solve most if not all of the WS-* or WS-SCRAM issues.