A little background on why you should care: AtomPub is a REST protocol for creating and updating "things" on the web. The primary use case is content management - blog entries, wiki pages, anything with a title and content - but it's really very general.
Sounds handy, but I'm still having trouble caring all that much. What's something actually interesting that people do with it?
Google's GData API is based on it. Word supposedly has an AtomPub client, so you can publish right from your word processor. It's the next step in reinventing USENET. Et cetera.
The tech isn't particularly exciting, but it provides a way to unite what are now totally different APIs.