If Microsoft wants to really contribute something useful in the spirte of SmartTags why not go with the Annotea project? It is a developing annotation standard by the W3C. Some tweak of this idea should be able to provide the same functionality in a way that site creators can opt in on and others participate in without having to bow to the wills of the evil empire.
Then again maybe I’m just too much a leftist commie for a world like that.