So I hacked Radio to accept an Atom 0.3 feed. Or did I? Well, I subscribed to the feed using Radio's standard mechanism, it's internal webpage. It reported an error with channel title, but a page refresh showed it in my list of subscriptions.
To be sure, I opened aggregatorData.root and deleted Mark Pilgrim's conventional feed. Right above it in the table was the atom-based URL. I'll leave it there and see how Radio parses it.
By the way, his Atom feed validates at Userland's RSS validator. [fingers crossed–maybe the aggregator won't know the difference.]
update: It knew the difference. So, I googled for some help and found a page about Radio's driver architecture that had some good information. Great, no problem. I followed the instructions there and was rewarded with disappointment: no compile script. No problem–more googling found this post that Dave wrote about his creation of a format reader for an Echo feed. Fine, Dave, but why didn't you follow your own instructions? Your feed format compiler is in the system space reserved for Userland. The original aggregator driver architecture post says it's in the user space. What gives?
Also, Mark tried it and it worked, but the format changed. I don't have the experience to catch up. Anyone else want a go?
This just points out how “pointless” the whole feed format this is. It's like Word vs. Word Perfect. Who cares?