Wordpress 2.0 does not natively support Atom 1.0. Fortunately, there is a patch that can be applied to wp-atom.php to get you most of the way there. There were a few other changes I had to make; I can mention those in the comments if people are interested.
You can verify that the Unto.net feed is valid Atom 1.0.
I resyndicate the Unto.net feed through Feedburner. When users subscribe to Unto.net they do it via my Feedburner URL:
http://feeds.feedburner.com/untonet
You can also verify that the Feedburner feed is valid Atom 1.0.
FeedBurner offers a service called "SmartFeed". SmartFeed automatically translates your feed into a format that it expects the client to understand.
But even SmartFeed isn't working for a surprising number of feed readers. I think that most feed readers can actually understand Atom 1.0 in part, but several have a very hard time with elements of type="xhtml". Of the readers I tested only My Yahoo and Safari handled type="xhtml" correctly. Bloglines, Google, and Firefox did not.
I went through the trouble of getting Unto.net to output unencoded XHTML. Unfortunately since the majority of readers can't handle it I had to go back and change everything to opaque encoded HTML.
James Snell recently wrote a post entitled "what does your feed reader show?" in which he gives a sample Atom 1.0 feed to be tested in your favorite blog readers. Most blog readers seem to mess up on Atom 1.0 somewhere.
This is unfortunate, as Atom 1.0 is a much better format from the perspective of the client. Atom 1.0 is unambiguously and clearly defined. The inclusion of explicit content types makes it possible to build all sorts of rich applications using Atom.
I'm not sure how long it will take publishers and clients to start supporting Atom 1.0. But as far as I'm concerned the time to start is now...