Apple has apparently included a blacklisting mechanism in iPhone OS 2.x through which the device can phone home, check for unauthorized applications, and disable them. The OS includes a URL that points to a page containing a list of unauthorized applications, specifically those listed here.
According to Jonathan Zdziarski, author of the book iPhone Open Application Development and an iPhone forensics manual:
This suggests that the iPhone calls home once in a while to find out what applications it should turn off. At the moment, no apps have been blacklisted, but by all appearances, this has been added to disable applications that the user has already downloaded and paid for, if Apple so chooses to shut them down.
I discovered this doing a forensic examination of an iPhone 3G. It appears to be tucked away in a configuration file deep inside CoreLocation.
Originally posted at iPhone Atlas.
Editors' note: For more information, see Tom Krazit's follow-up post, "Much ado about the iPhone's 'kill-switch'."