Updated to add link to new PC Magazine article
AntiSec - the hacker group that is the "merger" of Anonymous and Lulzsec - claims to have obtained the unique device identifiers (UDIDs) from 12 million Apple iPhone and iPad users by breaching an FBI computer, and have published more than 1 million of them.
Details of the hack can be found at ZdNet , Slate, and The Washington Post. According to the hackers, the alleged hack was intended to publicize the existence of some kind of secret FBI tracking project, also raising an embarrassing question of security for the FBI.
If you want to check whether your Apple UDID was in the compromised file, The NextWeb has developed a nifty quick check tool that you can see here.
UPDATE: PC Magazine has published an article analyzing how the hackers might have obtained the UDIDs associated with the Apple devices. Good reading.