Many of the responses so far seem to assume that the server's output will be consumed by a web browser, and be processed with _javascript_. But if we're talking about "services" in general, this is far from the only case.
Indeed, it's partly because JSON parsing returns an anonymous object, whereas XML parsing messes with the DOM, that JSON is easier to use for arbitrary interaction. But if you use James Clark's MicroXML parser, you can consume a stream of (Micro)XML and turn it into similarly anonymous objects.
