I've been doing XUL work in XEmacs, and it has been pissing me off that xml-mode wasn't doing indenting or syntax highlighting automatically. I finally asked Nathan about it, and we figured out this addition to my init.el to make working with XML and XUL files indent nicer:
;;make xul files start in xml mode
(setq auto-mode-alist
(append '(("\.xul$" . xml-mode)) auto-mode-alist))
;;make xml-mode indent
(add-hook 'xml-mode-hook
(lambda ()
(setq sgml-indent-data 'T)))
Much, much, much better.