Saying it had found that many of the unlocking programs "cause irreparable damage to the iPhone's software," Apple spelled out the policy. "Users who make unauthorized modifications to the software on their iPhone violate their iPhone software license agreement and void their warranty," the company said in a statement. "The permanent inability to use an iPhone due to installing software is not covered under the iPhone's warranty."

Several unlock hacks are circulating, including the free open-source anySIM unlock created by iPhone Dev Team, and iPhoneSIMFree, a commercial unlocking program that sells between $45 and $90 online. The unlocks let users insert a SIM card programmed with account data from another mobile provider so the phone can make calls on that carrier's network, rather than solely over AT&T's.

Other mods, or modifications, let iPhone owners install their own ring tones -- Apple charges 99 cents on top of the usual 99 cents tune fee for a ring tone -- and add new software to the Apple-approved applications on the device.

Apple also said it would issue an iPhone update this week, and hinted -- as expected -- that it will use firmware updates to cripple unauthorized modifications. "Apple has discovered that many of the unauthorized iPhone unlocking programs ... will likely result in the modified iPhone becoming permanently inoperable when a future Apple-supplied iPhone software update is installed," the statement said.

The coming update has been no secret. Shortly after Jobs unveiled the new iPod line and announced the $200 iPhone price drop that angered many early buyers, Apple confirmed that it would update the device to add new features, including the ability to buy tracks from the iTunes music store over Wi-Fi.

Not surprisingly, Apple's policy didn't scare off those who have modified their iPhones. On the company's iPhone support forum, a user identified as ansuz07 said, "The percentage of iPhones that have become bricked from hacks is very low. Even those that experienced problems could be fixed by a simple restore. Apple is going to make it sound a lot worse than it actually is since they are the ones who don't want you to do it in the first place."