I've just been using the XML editor that's part of Eclipse Ganymede. The original version of that one had a serious problem, however - reformatting the XML caused it to drop quotes and display false error messages. A recent update has repaired that.
The editor is supposed to be able to do tag/attribute hinting based on the DTD/schemas indicated. I haven't had much success with that, lately.
An IDE is no substitute for an Intelligent Developer.