True to its warning, Apple Inc. issued a software update Thursday that the company said would create problems for iPhones that have been modified to use a cellular carrier other than AT&T Inc.

Apple warned earlier this week that the iPhone update — which adds access to the iTunes Wi-Fi Music Store and fixes some security flaws — could wreak havoc or permanently disable phones that carried unauthorized programs designed to unlock the handset from the exclusive use of AT&T's network.

Several gadget enthusiast Web sites, including Gizmodo and Engadget, as well as online postings from hacker communities reported that, depending on which unlocking program was used, certain modified phones no longer worked after they installed the software update.

In some cases, the phones would work, but only with the original SIM card that ties the phone to AT&T.

• Click here for FOXNews.com's Personal Technology Center.

Some sites also reported that iPhones equipped with other unofficial, third-party applications became disabled after the update.

A tech support representative from the hacker Web site iPhoneSimFree.com said in an e-mail that its development team found Apple's software update did not appear to be an intentional offensive move against hackers.

Programmers found that older Apple software updates also created problems with the unlocked iPhones.

"It was simply an inadvertent result of a pre-planned update," said the representative who went only by the name of "Jim" and declined to identify himself.

Some hackers had characterized Apple's warning as "a scare tactic."

Despite Apple's history of playing cat-and-mouse games with hackers concerning other products, Apple officials insisted they were "not proactively" trying to make hacked iPhones useless.

[The process of "unlocking" a cell phone — modifying it to accept service from a different carrier than the one it was designated for — is currently in a legal gray area in the U.S. A Library of Congress panel decided in November 2006 that a consumer had a right to unlock his personal phone.]

It was unclear how many iPhone owners had unlocked their phones, but the programs — including several that can be downloaded for free — appeared to be particularly popular with European consumers.

Apple isn't selling the iPhone or initiating the service in Europe until November, so the unlocking software allowed Europeans who bought iPhones in the United States to use the phones.

Installing Apple's latest iPhone update is optional.