Couple years ago Charles Petzold wrote about C# Application Markup Language - XML syntax for C#. Go read it if you've not seen it yet.
Well, it was a fool day's joke of course (the publishing date at the bottom hints about it).
But yesterday, when playing with Windows Workflow Foundation rules, I've looked at the ruleset file, and discovered exactly that - huge XML file that looked familiar :), very close to Charles joke. This is how WWF internally stores the simple one-line rules that one types in the editor. Luckily for us, we don't have to type this XML, and WWF convert the human-readable C# into this monstrosity itself, so it is more an internal details than a language humans would ever use.