Mark has some good info on Atom and how to work with it. The example is making a linkblog in Atom, but the info there is also useful for learning how to work with Atom period. For those who don't know, Atom is an alternative XML syndication format "competing" against RSS.
Of course, my opinion of the whole RSS vs. Atom thing is that it's egos competing and that it really doesn't matter, but that's a different issue :-)