Phil Windley writes this compelling piece on why SOAP may be on the way out as a top choice of transport mechanism, being trumped by the simpler REST (XML over HTTP) formula. Carlos Peres is quoted as observing that the SOAP envelope is "extra machinery" that "has never been shown to improve interoperability."
This is one of a growing chorus of statements that REST is prevaling. Still many Web services developers swear by SOAP, especially since it's supported by the the vendors. The prognosis: it's likely we're going to be seeing both approaches for some time to come, depending upon the implementation.