(I could have sworn I did this earlier today! ;-).

A new feed spec is announced, this time by Tim Bray: Atom 1.0:
"It’s cooked and ready to serve. It doesn’t have an RFC number yet, but this is officially Atom 1.0 (HTML here). Here’s a comparison of RSS 2.0 and Atom 1.0. The implementors are hard at work. I’ll update this page regularly with Atom-related news and pointers, send word if you want yours included. Read on for crunchy Atomic goodness."
So, if you're using Atom 0.3, time to move on to Atom 1.0 or RSS 2.0 (see comparison here).