But... SDK4 introduced Cocoa's tree-based parser, NSXML, to the iPhone SDK, which is much easier and more pleasant to use in a Cocoa app. As a result, it might make sense to focus my energies on other iPhone technologies.
If you want an introduction to NSXML, you'll have to look in the Core Library docs, the iPhone docs have not yet been updated with these classes. I do not know yet if there are significant changes between the two versions or not. You can also check out this article over at MacTech from a few years ago written by yours truly which talks about using NSXML. It might be a little outdated at this point, but should give you at least a conceptual grounding. If I find they have completely change NSXML since then, I'll let you know; I'm going to try to find time today to play with the iPhone version of NSXML.