It’s a match between Apple and iPhone-unlockers. These guys keep locking and unlocking the iPhone, but it seems that Apple has dealt the final blow. It released an update to the iPhone firmware, that turns your iPhone into an iBrick (courtesy some git at ZDNet or PCWorld), if your iPhone is unlocked. You can’t use the thing any more. I don’t exactly know how it becomes unusable, but is apparently does. It’s best if you don’t connect it to your PC at all.
To the uninitiated, the iPhone is currently under a 2 year contract from AT&T. So some people want to jump that, and ‘unlock’ their phones from the contract so that they can use other carriers. Apple and AT&T don’t want that. Also it’s rumoured that AT&T forced Apple to take the move to release the update.