I just added an Atom 0.3 feed to this site (in addition to the RSS 2.0 feed which I'll keep as well). Following the example template from Mark Pilgrim, it was quite simple. The template uses a couple of MovableType plugins (UTCDate and MTEntryModified, both from Staggernation.com). For now, I didn't take the additional step of serving the file up as MIME type “application/atom+xml”, instead just sticking with xml type until aggregators better support the subscription model.
It will be intesting to follow the Atom development process to see what kind of new uses people find for it (beyond just replacing RSS).