The Patriot Act provides for secret warrants to spy on ISPs' customers. These "Section 215" warrants come with gag orders that mean that the company can't disclose their existence. This lack of transparency is ripe for abuse and is bad for ISPs' business. Apple is fighting back with a "warrant canary": they've published a transparency report (PDF) that states "Apple has never received an order under Section 215 of the USA Patriot Act. We would expect to challenge an order if served on us." If they are served with a 215 order in future, their next transparency report will drop this language, omitting any mention of 215, and keen-eyed watchers will know that they've been subjected to a secret order. I proposed a more ambitious version of this in September, though I was hardly the first person to suggest it. Good for Apple for using it.