(Editor's note: David Wang is an accomplished iPhone hacker and member of the iPhone Dev Team. Tinker with your gadgets at your own peril--we're not responsible for what happens if you brick your iPhone, however unlikely that may be.)
Maybe you want to liberate your iPhone from Apple's clutches. Maybe you just want to tinker with something new. Either way, you've seen Android running on the iPhone, and you want to try it for yourself.
Still a Work in Progress
Although this port does everything that you expect your smartphone to be able to do, it isn't usable for day-to-day activities just yet--I haven't implemented any power-management functions, so a fully charged iPhone running Android will last only an hour or so.
A few bugs and performance issues remain, too, so while the phone will be usable, it won't be fast. If you do something unexpected (such as forcing the iPhone off), there is a small chance that you may end up restoring your device. However, it is impossible for any bugs to brick or disable your iPhone permanently.
Finally, media syncing is not working, so loading your media onto your phone is kind of a pain. I'm working as hard as I can, though, and I expect to fix these issues soon.
The iPhone is a well-engineered device, and it is virtually impossible to brick if you know these techniques. If all else fails, remember that you can always restore using DFU Mode.
You'll also need to be reasonably comfortable working in a command-line interface, and unless you're confident in trying to compile your own binaries, you'll need a PC running Linux (or a Linux virtual machine).
What You Need
1. A first-generation iPhone or an iPhone 3G with firmware versions between 2.0 and 3.1.2, jailbroken with Redsn0w, Blacksn0w, or PwnageTool. If you already updated your handset to 3.1.3 or to a 4.0 beta, you must use PwnageTool to create a jailbroken 3.1.2 .ipsw file to restore down to.
Note that I am explicitly excluding the iPhone 3GS, all iPod Touch models, and the iPad. This hack will not work with those devices (yet). I am also explicitly excluding iPhone OS 3.1.3 and all of the 4.0 betas. It will not work with the Spirit jailbreak, either.
If you haven't yet jailbroken your iPhone, don't worry--it's a simple process that consists mainly of pressing buttons on the device when prompted and clicking the next button in a wizard. I humbly recommend Redsn0w, since I wrote much of the code for that program.
Although much of the process can be conducted on any machine, one of the tools involved (called 'oibc') has not yet been ported to Windows. In addition, the binaries I provide are compiled on a 32-bit Ubuntu machine.
All of the utilities compile for Linux and Mac, however, so if you're feeling adventurous, compile the sources at github.com/planetbeing/iphonelinux and github.com/planetbeing/xpwn instead of using the binaries.
4. The iPhone OS 3.1.2 .ipsw file for your device, namely either iPhone1,1_3.1.2_7D11_Restore.ipsw or iPhone1,2_3.1.2_7D11_Restore.ipsw. Chances are, you already have this file somewhere on your computer, but if you need it, you can download it.
5. The firmware for the Marvell WLAN chip inside the iPhone. Go to the URL, and on the right side of the page you should see a drop-down menu labeled 'Choose your platform'. Select Linux 2.6 - Fedora from the drop-down menu and click the Search button underneath. Download the file labeled SD-8686-LINUX26-SYSKT-9.70.3.p24-26409.P45-GPL. You'll get a file called SD-8686-LINUX26-SYSKT-9.70.3.p24-26409.P45-GPL.zip.