I was going to post this as a comment to the Blogging In Flock post, but I see you've closed comments on that post. It looks like Elias Torres has implemented an APP server for WordPress. I haven't tried it, but it looks interesting, and Elias usually makes good stuff.
Thanks for pointing that out, Darren. I had noticed that as well and look forward to seeing if it works.
The real trick will be getting Atom Publishing Protocol support merged into the main WordPress tree. With stuff like this it is all about priorities, and as evidenced by the difficultly in getting valid Atom 1.0 support in the WordPress mainline, (note that my migration back to stock WordPress broke my patched Atom 1.0 feed), it is a matter of whether or not the right people find it important.
Fortunately, the momentum is strong for APP. Google's GData API uses an exchange protocol based on APP. And yesterday's launch of a Google Base API layers another incredibly powerful application on the protocol. Blogger.com is moving exclusively to APP (and I just noticed Google also recently released a GData API for Blogger). That move is prompting Windows Live Writer to support APP in the near future as well.
Speaking of Windows Live Writer, and by association, Windows Live Spaces, I think people need to step down from the rhetoric around what constitutes a "blog." In my opinion, Writer and Spaces are the most obvious signs that these technologies are now part of the mainstream. You could certainly make a case that there are more hard-core publishing tools designed for the hard-core publisher, but slow down for a second and get some perspective. The 99% case of normal human beings are never going to be "hard-core" about personal publishing. They don't need or want the tools that you and I do.
Besides, I love that Microsoft is getting so deep into the personal publishing game at all. Think about it; the world's #1 consumer software developer, a company with by far the most "touch" in the technology world, is now bringing technologies that we know and love to a much broader audience. Will they always do it in a way that meshes exactly with what I personally need? Of course they're not going to, and they shouldn't be targeting me as a consumer. Will I try and influence that direction? Hell yeah, but I won't mistake my own personal requirements for that of the majority.
And if you're reading this post, then you too are in the miniscule minority in terms of target demographics.