With the launch of Google Base, Google has created a new RSS extension. The RSS extensions facilitates bulk uploads of classifies to Google Base and supports RSS 1.0, RSS 2.0 and Atom 0.3. I'm unsure why they didn't do Atom 1.0. Could've been a timing thing. Their development complete date may have been too near the Atom 1.0 finalization date. Yet another reason to have gotten Atom 1.0 done earlier (is anybody using it yet?).
Dave Winer asks "Where's the API?" It's RSS Dave. Remember, you helped create it.
Where's the API? You can put data in - how do you get it out?