I asked this question a few weeks ago and someone posted a link to an Apple employees discussion of this topic. Unfortunately, I can't remember the link, but you can probably search the boards to find it: I started the thread and it was about two weeks ago.
The bottom line is that Apple is indeed developing Cocoa classes for dealing with XML and it will be in a subsequent OSX release. The Apple employee refused to respond to questions regarding timing or more details (understandable). Anyway, they seem to have a plan.
kman