Well, I'm excited to report that we've landed the first iteration of this feature into Firefox nightly builds (the pre-beta builds that are rough around the edges)! If you'd like to try out the feature, grab a nightly build; I must warn you though, these nightlies are not as stable as the beta releases.
In the build, to enable the feature, open the preferences pane and select the advanced tab. Tick the box that says "Tell sites I do not want to be tracked" and start browsing.
Every connection your browser makes to download content will send a signal that says "don't track me." Literally, it looks like this to servers:
Note: this is different from the initial experiment that used "X-Do-Not-Track" and my original post last week that said "Tracking-Preference: do-not-track"; it's both shorter and very precise. The researchers at donottrack.us are also recommending this syntax.DNT: 1
I encourage you to try out the test builds, or if you'd like to wait for a more stable version, wait for an upcoming beta release with the feature in it. We do not anticipate that sites are looking for the signal yet, so you probably won't notice a difference as you browse the web. I'm hoping to have a demo site available shortly that will give you an example of what types of changes you might see using this feature -- and when I do, I'll post a link here.