Re: ISSUE-27: "opt back in"

From CDT's v2 definition [1], this paragraph is particularly relevant:

> CDT also believes that a userʼs decision to enable a DNT mechanism can be overridden upon a 
> grant of affirmative and specific permission from the user in response to a clear and 
> conspicuous prompt from a service.  For example, a news site could present a dialog box and 
> request that the user grant permission to a certain ad network to track her on that site as a 
> condition of service. Or a photo-sharing site could offer users a choice during registration to host 
> limited amounts of data for free or to host more if the users allow certain third-party tracking. As 
> long as the request for permission is clear and prominent and a user is given the opportunity to 
> make an informed choice about the value proposition, companies should be able to obtain a 
> userʼs affirmative permission to override the generic DNT instruction

[1] http://cdt.org/files/pdfs/20110447_DNT_v2.pdf

(This is also a test of Tracker and an example for how to contribute relevant input document text to a particular issue.)