Given that Joe works for Google on GData, I have assumed that Joe's post is Google's attempt to float a trial balloon before extending AtomPub in this way.
As I explained in the comments on Dare's post, this is my personal blog and unless otherwise stated, my own thoughts and ideas. If I weren't just speaking for myself I would make that clear.
Like I am about to do now.
At Google we are considering using PATCH. One of the big open questions surrounding that decision is XML patch formats. What have you found for patch formats and associated libraries?