Reading Jon Udell's weblog, I ran across the Xopos XML editor. I clicked on the demo and within minutes was editing XML inside my browser. I haven't played with it extensively, but what I did do was pretty neat. The editor is fairly comprehensive; you can edit the content of cells and move them around (subject to the schema) without ever seeing the XML or even knowing what XML is. It was smart enough to warn me when I left the page with unsaved changed (something I've fussed with in other browser based editors). If you've got data you want entered or edited in an XML structure, this may be a good tool to look at.