I talked about executable XML in this blog post called More language focus a few weeks ago. Niclas Nilsson pointed me in the direction of this great blog post by Charles Petzold yesterday:
C# Application Markup Language (CSAML): An Evolutionary Leap
Oh, and while you're at it, you should definitely subscribe to Niclas' blog of course!
Update
Sorry, I should have made it clear that I was aware of that Charles joked. Sometimes blog posts don't have high enough bandwidth, especially not for irony.
:-)