A couple colleagues of mine here at IONA, Frank Lynch and Mark Fynes, have just published an article over at the WebServices Journal entitled "Where HTTP Fails SOAP." It discusses scalability problems that can arise when applying SOAP/HTTP in the enterprise back end. Given that it seems that many people just assume that SOAP/HTTP is the end-all and be-all of protocols, I think Frank's and Mark's article provides some unique and useful insights into where and why SOAP/HTTP can come up short.
