John Evdemon over on MSDN suggests that XML is not code and thus cannot be open-sourced. I'm afraid that's a rather narrow view: XML is text, and thus it can be code, or data, or a document or document schema.

• code can be open-sourced; for example XSLT is simply code (programming languages can be declarative, procedural, logic-based, or object-oriented); some XML documents can host code (for example XUL or XAML) in which case it can also be open-sourced
• a document can be licensed (there are several open licenses such as the OpenContent License (OPL) Open Publication License or the GNU Free Documentation License (FDL)
• a document schema can be licensed or an open standard such as the OASIS Open Office format
• data can be free or may be published under a license that restricts its use

Here endeth the lesson ;)