DNT is pretty laughably ill conceived. Vague definitions of what is able to be tracked, what constitutes a first party and third party (Is a browser vendor first party or third party?). Some of the major proponents are companies like Google and Microsoft, both with their respective gigantic ad platforms DART and Atlas. Would they be able to collect data directly from the browser and dominate the ad world? It's certain that if this were legislated that it would destroy the business model for many large companies. (Yahoo, ValueClick, AOL).
The existing legislation requires an opt-out policy from tracking. Advertisers must respect the user's choice with regards to targeted advertising. Does Microsoft turning on DNT by default represent the users choice or does it represent Microsoft's interests? What about third party identification for things like disqus? Sure we have ways to handle CORS today but what if there was a law (that only effected the USA) saying you were not allowed to "track" if you were a third party? If it's just about advancing web technologies why isn't this just a w3c proposal or RFC? What's wrong with the existing opt-out methods?
From what I see DNT is just a political tactic to gain power in the ad-world, not a way to protect the privacy of users.