See http://lists.w3.org/Archives/Public/public-webapps/2011JulSep/1622.html for the proposed API.
This is a meta-bug, as the implementation is expected to be split into several patches.
The interface definitions are now listed in the DOM4 spec, which is a bit more readable than the mailing list archive:
(Just to update the URL for the Mutation Observers href in the DOM4 spec):
This can be considered done at this point.