Currently there are very few sites that have implemented the do not track header. Right now it is more of a proposal than anything else. You will most likely find that your favorite sites show up with a big red X and a list of 3rd party tracking sites. However, if you are a developer or web master and want to get your website in shape, this page is a great way to test.
There are also very few browsers that send DNT. Mozilla added the option to Firefox Version 4 Download