TechCrunch reports that Apple, Inc. has slipped a "warrant canary" into a recent report:

The very last line of Apple’s report today states “Apple has never received an order under Section 215 of the USA Patriot Act. We would expect to challenge such an order if served on us.”

The cleverness of this becomes evident when you realize that if it had received such an order, it could not disclose it under current rules surrounding national security orders for user data. This tactic of announcing ‘nothing’ with regards to a government subpoena for data is known as a kind of ‘warrant canary’. Basically, Apple says that at this point it has not received any such order. But, if that phrase stops appearing in future transparency reports, this acts as a ‘canary in a coal mine‘ that indicates to users that it may have been forced to comply with such an order and not disclose it in the future.

I think this is clever.  While the Patriot Act would not allow a company to affirmatively report that they have been subpoenaed, there's no prohibition on a company reporting that they have NOT been subpoenaed.   But Apple isn't the first to do this.

Civil Liberties attorney Matt Cagle notes that Lookout Security has also recently stated they’ve never received a national security order for user data.

This tactic was used by offsite backup company Rsync in what is believed to be the first commercial company application. 

Major email providers like Google, Yahoo and now should weekly or monthly send users an email reporting the lack of a subpoena.  Once the emails stop coming, the user would know that his privacy has been compromised.