<meta name="canary" content="2013-11-17"
With this is would be possible to build into browsers or browser extensions a means of alerting users when a company has intact received such a secret warrant. Similar to the actions taken by Apple recently.
The advantage the metatag approach would have its that it would not require the user to search out a report by the company in question but would show the information upon loading of the page. Once the canary metatag was not found or when the date of the canary grows older than a given date a warning could be raised.
Several others have proposed similar approaches including Connor Friedersdorf in the The Atlantic (http://www.theatlantic.com/politics/archive/2013/09/interent-superhero-corytorows-plan-to-thwart-the-nsa/279540/%20rel=) and Corry Doctorow's Dead Man's Switch (http://craphound.com/?p=4932%20rel=)