Owners of hacked iPhones have begun posting instructions on how to roll back a recent Apple firmware upgrade that rendered their mobile phones unusable.

The instructions were available Monday on the iPhone Dev Wiki, a Web site devoted to iPhone software hacks and tools.

Since the iPhone's launch, enthusiasts have been developing ways to allow the devices to run unauthorized software and to unlock them so that they can be run on any mobile network. Late last week, however, Apple cracked down on these efforts by releasing a software upgrade that made hacked iPhones unusable.

Since that release, however, hackers have been working on techniques that reverse the effects of this upgrade.

These latest instructions allow users to roll back their firmware upgrades and use some functions such as the phone's iPod and Wi-Fi capabilities, but they do not necessarily restore the phone's ability to make calls, according to the iPhone Dev Wiki. That's because hackers have not yet found a way to roll back the firmware used by the iPhone's baseband chip, which is used to make calls.

"So far all attempts to downgrade the baseband have been unsuccessful," the Wiki said. "There have been several reports of successful baseband downgrades online, but these haven't been confirmed. "

Another major area of research has been into techniques that can unlock iPhones that are running the latest 1.1.1 firmware.

But because Apple has now done a much better job of encrypting its iPhone firmware, this will be a much harder job than it was the first time around, said Tom Ferris, a security researcher who works on hacking the iPhone.

"That's what everybody's working on right now is trying to get into the firmware," he said. "Just like Steve Jobs said, it's a cat-and-mouse game."