Software Development Online: Errant Architectures: "If both sides of the wire use the same binary mechanism, an XML interface doesn't buy you much other than a jazzier set of acronyms." (registration required at this site, I believe)

That's well said. XML is most appropriate (in my mind, read "only appropriate") when you're publishing for an anonymous consumer. That's pretty rare, unless you're a news site or giving out public data, etc. If you're not dealing with someone anonymous (and I mean completely anonymous), there are nearly always better ways. The above article (section called "Interfaces for Distribution" on that page) is a good read why XML is overkill and a performance trasher.