The XML standard defines a mechanism for encrypting the data in a XML document. It's part of the standard.
Does anyone use this XML encryption format? Are there any significant deployed systems that use XML encryption? Is it used in practice, or is it just something that exists on paper and is pretty much irrelevant in practice?
(Note that XML encryption is different from sending an unencrypted XML document over SSL. The XML encryption mechanism provides a way to encrypt the contents of the XML document, so you have an encrypted document.)