I was working with yet more XML the other day and found out after almost completing my testing in the simulator that NSXMLDocument and support classes are not included on the device. This is apparently because they are a little too heavyweight for the device. I find that odd given that we can deploy complex 3D games on the same platform yet XML isn't quite ready. Anyway, there's a couple of alternatives. We have access to the libxml2 libraries which would need to be included in the build if used. There's a library in google code for TouchXML which should help with this. Then there's the included XMLParser which works as an event based parser reading through the tree. I suppose the third option (even though I only said a couple) would be to use CFPropertyListWriteToStream which should sort out some woes too.
UPDATE: All sorts of stuff has changed for OS 3.0 so I will get around to updating this page asap.