Apple doesn’t support the installation of third-party applications on the iPhone, but the enterprising developers at NullRiver have created a program called AppTapp that will let you install including chat clients, games, and other pieces of software – including those that are needed to unlock your iPhone. (For more detailed instructions on using AppTapp, see our iPhone Hacking Kit.)

Download and install AppTapp. Once you run the program, the first thing it will do is ask what version of the firmware you’re running. If you’ve just updated your iPhone, you probably have Firmware 1.0.2-select that and click on Continue.

Assuming all goes well, you should get a screen that begins with the words: “Congratulations! You have successfully installed AppTapp Installer onto your iPhone.” You should then find a new application on the iPhone’s home screen called Installer.

The next instructions will require downloading some applications from the Internet to your iPhone-so make sure your iPhone is connected to a WiFi network. Launch the Installer program, and you’ll see a long list of various applications that you can install. Scroll down, then select and install-in order-Community Sources, BSD Subsystem and OpenSSH.

Perform the GSM Unlock

First, download and install Unlock.app, lockdownd, and Cyberduck.

Then go to the iPhone’s General settings and set Auto-Lock to Never. Next, you’ll need to get your iPhone’s IP address on your local network. To do this, go to Settings: Wi-Fi and click on the blue arrow next to the network that you’re connected too. Write down the IP address, as you’ll need it for later.

Now, open Cyberduck and click on the Open Connection button in the top left of the window. In the connection window, type the IP address that you just noted. Make sure that SFTP (SSH Secure File Transfer) is selected, and that you are using Port 22. For the username, type root, with the password of dottie.

Once connected, navigate from /private/var/root to the main directory by choosing the / item. Next, navigate to /usr/libexec and drag the file named lockdownd (the one you downloaded earlier) into it. It will ask if you want to overwrite the existing file-click on Continue to do so.

Now, in Cyberduck, drag the Unlock file from the Mac’s desktop into the /Applications folder at the top of the main window. Once that’s done, turn off your iPhone and remove the SIM card, then turn your iPhone back on and choose the application on your iPhone called Unlock.

Assuming everything went smoothly, there should be a white screen that says “All files found. Ready to go. Press Start.” Once you press start, it will take about 20 minutes for all the files to install.

The progress messages will appear at the top of the screen as it progress-once you see “Completed” then you’re finished. Insert a new SIM card of your choosing into your iPhone. Congratulations, you’ve just unlocked your iPhone!

(If you’re planning on using a T-Mobile SIM card in your iPhone, you’ll want to modify the iPhone’s settings. Follow the instructions at the bottom of the ModMyiPhone tutorial to make sure the settings you enter conform to your T-Mobile data plan.)

The aftermath

One Macworld editor tried this approach – minus the faux-activation step – on his iPhone and was able to insert a T-Mobile SIM card from a co-worker and get the iPhone up and running on the T-Mobile network, as if it were her phone.

(To swap SIM cards in your iPhone, insert an unfolded paper clip into the small hole in the top of the iPhone, next to the headphone jack. This will open the iPhone’s SIM card slot, allowing you to remove your AT&T card and replace it with a different one.)

We don’t know if future Apple iPhone updates will break this functionality. But if you’re about to embark on a European vacation, or if you avoid downloading those iPhone updates, this method will allow you to use the SIM cards of other cellular providers in your iPhone, provided that those providers also use the GSM cellular radio frequencies.

[Cyrus Farivar writes for MacUser, The Economist, National Public Radio, Slate, and others. He is proudly using his new iPhone on T-Mobile's network in Oakland, California.]