In reaction to The S stands for Simple, Nelson Minar has some very choice quotes. These are all the more pertinent since Nelson has hands-on experience with real world deployments of SOAP services:
As someone who bears some past responsibility for well used SOAP services (Google's APIs for search and AdWords) let me say now I'd never choose to use SOAP and WSDL again. I was wrong.
...they boil down to massive interoperability problems. Good lord, you can't even pass a number between languages reliably, much less arrays, or dates, or structures that can be null, or... It just doesn't work.
The deeper problem with SOAP is strong typing.
This backs up my experience; if you don't have control of both ends of the wire then loosely typed documents beat strongly typed data-structure serializations.