It seems I never got around to posting about mutation observers here (MDN has a couple of pointers elsewhere). If you are using mutation events in any of your applications now is the time to start looking into how you can migrate away from them. The latest Firefox offers a first-class implementation of mutation observers and Chrome has one with ugly prefixes.
The long term plan is to remove the mutation event implementation from browsers and only support mutation observers going forward. This transition will be somewhat painful, but should tremendously help with keeping DOM implementations relatively simple and easy to hack, as well as giving applications a much better platform to work with.
(See also DOM Core gains an Events chapter and A new low-level DOM mutation notification API.)