It's both. The "DNF" header can't be a serious proposal since it doesn't follow the convention of other HTTP headers. They're leaving "Do-Not-Fool" unused in case it later becomes a serious proposal. But it does actually send the header, so in that sense it's legit.
The rationale for "DNT" is to minimize the bandwidth/latency impact of adding a new header to every single HTTP request (for users who enable the "do not track" option).
Announcement for our DNF support: http://ourdoings.com/2011-04-01