One of the (many) blogs I track is that of Wouter Van Vugt. It's a great resource for all the technical issues and goings-on in the wonderful world of Open XML. I even had the pleasure of meeting him at the Office Developer Conference in San Jose, where he obligingly autographed a copy of his book for me.
Wouter's post on Open XML's "Custom XML" feature effectively combats the FUD that is being spread on this issue.
He explains, in clear, concise, technical terms, what "Custom XML" support really is and separates the FUD from the Facts.
Brilliant stuff! Couldn't have done it better myself!
:-)