This is how Mark describes it in the semantics section of his site:
rel="noreferrer" “indicates that no referrer information is to be leaked when following the link.” No shipping browser currently supports this, but support was recently added to WebKit nightlies, so it will eventually be showing up in Safari, Google Chrome, and other WebKit-based browsers. [rel="noreferrer" test case]This is an interesting use of rel in HTML, changing the behavior of the browser. Especially a default behavior like this that has been around since the stone age (early stone tablets had a referrer of course...).
The impact to the Analytics community could be quite far-reaching. Referrer information is obviously quite important to a lot of base reporting dimensions. We'll all be interested to see if this specification will actually make it into HTML5.