We're considering using XML as the foundation of a messaging system that would support communication across multiple platforms and be accessible in both Java and C++. Our leaning is toward having the core of the system be in Java. It looks like there are many XML messaging efforts going on at very different stages of maturity. Is there one system that is gaining prominence? Is there a single site that's best for getting info on using XML for messaging?
I think the UDDI/SOAP area is looking pretty strong. All the major players seem to have signed on. I use the Apache/IBM toolkit at http://xml.apache.org/soap/index.html - as far as the litmus question "How much of this stuff is being used in production corporate apps?" - I really don't know. Microsoft also has a SOAP toolkit, but I think they only support their IIS web server. It can be downloaded from their corporate site. Have fun!