I had a talk about Atom and extensibility with Dave Orchard this morning, and he convinced me that there is benefit in a must-understand facility, but then educated me as to how complex it can be to implement.
Based on that discussion, I have just published PaceMustUnderstandElement and PaceExtendingAtom. Note that the WG could reject PaceMustUnderstandElement and I think that PaceExtendingAtom would still work.
For convenience: http://www.intertwingly.net/wiki/pie/PaceMustUnderstandElement http://www.intertwingly.net/wiki/pie/PaceExtendingAtom
-Tim
