Interesting timing: just as ATOM 1.0 is finally becoming an IETF/W3C standard, Microsoft publishes her extension to RSS 2.0 here. While the Microsoft extensions are licensed under the Creative Commons ShareAlike license and Microsoft also seems to pledge to not apply royalties to implementors, RSS 2.0 is still under copyright from Harvard and cannot be changed at this point.
It will be interesting what Sam Ruby and Tim Bray will have to say about this. Meanwhile, you can take a look at Sam’s RSS to ATOM comparison.