DNT is supported by all browsers but is pointless for now
With Google Chrome finally implementing support for Do Not Track, all major browsers now offer or will offer in the near future the ability to make your preference about ad tracking known.Being able to opt-out of ad tracking with a couple of clicks seems like a major victory for privacy and, more importantly, for user choice.
But it's an empty victory if that's where it ends. Having support for DNT built into all the browsers is meaningless if no one respects it.
The idea behind DNT is to enable users to tell the websites they visit that they don't want to be tracked, i.e. they don't want a targeted ad profile being built based on the sites they visit, the ads the click on, the news they read, the games they play, the searches they do and so on.
But DNT doesn't enforce this wish, users can make their option heard but there's no guarantee that they'll be listened. Which is why the crucial part is getting ad companies to agree.
This is also why Google building DNT into Chrome is much bigger than, say, Apple building it into Safari. Google runs the world's largest online ad network and having it support DNT will be a major win.
Note that Google advertising doesn't support respect DNT and neither do the vast majority of ad networks and advertisers. Google also hasn't announced any concrete plans for implementing support in its ad network.
In fact, at this point, the list of companies that have said that will respect DNT is very short, the list of companies that already do so is even shorter.
Twitter committed to respecting wishes expressed via DNT, but that's the only major tech company to do it. For all its boasting, Microsoft has not pledged to respect DNT in its ad business and certainly doesn't do it now.