Erica Sudun, writer at TUAW, posted her guinea pig process of testing the 1.1.2. jailbreak. First you need to downgrade to 1.1.1 if you already have 1.1.2. Then you will have to prep for 1.1.2 and finally jailbreak. You can read about her experience below.

Starting in 1.1.1

“The key to the 1.1.2 jailbreak is to prepare your system in 1.1.1 before you upgrade. This meant I had to downgrade from 1.1.2.
I quit iTunes and connected to my iPhone using iPhuc. It’s not an especially recent version of iPhuc but it did offer two key features: enterrecovery and filecopytophone. Using iPhuc, I typed “enterrecovery”, let my iPhone do exactly that–enter recovery mode–and quit.

Next I unzipped my ~/Library/iTunes/iPhone Software Updates/iPhone1,1_1.1.1_3A109a_Restore.ipsw file. Despite the name, it is actually a zip file.

I relaunched iPhuc, and from there, I issued a filecopytophone command with one argument, the WTF from the unzipped Firmware/dfu folder, namely Firmware/dfu/WTF.s5l8900xall.RELEASE.dfu. To finish with iPhuc, I entered “cmd go”. I then restored my phone to 1.1.1 with iTunes and navigated to jailbreakme.com to perform the jailbreak.

Preparing the 1.1.1 iPhone

Remember that these are the same people who brought you jailbreakme.com. So don’t be surprised that preparing for the 1.1.2 upgrade involves nothing more than installing a single Installer.app package. Head over to the tweaks category and install OktoPrep. It takes just a second and once done, you see the message in the picture I show above. I tapped OK and quit Installer.app.

Next, I upgraded to 1.1.2. Since I manually downloaded the ipsw file last night, in iTunes, I option-clicked Check For Update. (Windows users use shift-click instead of option-click.) The update went smoothly without incident and I soon had a very nice 1.1.2 iPhone.

Jailbreaking

The next step uses the new software. I ran a command-line Mac version of the software but by the time you do this, it should have a lovely one-click user interface. The software connected to my pohone, uploaded Installer.app, SSH and the core files, and then read, updated, then wrote the flash image.

Finally, the software prompted me to enter a new password (up to 8 characters) for my unit. A few seconds later it told me that my process was done and that I should reboot. So I did.

The New 1.1.2 Screen

I was expecting Installer.app on my screen. What surprised me was the SSH Icon. Without any work, a simple SSH application appeared on my screen, making it easy for me to immediately enable secure shell access. I launched the app, switched OpenSSH from OFF to ON and waited about 30 seconds as it generated my keys.

Then, at the terminal command line, I removed my home .ssh folder–so I wouldn’t have authentication errors with my keys–and ssh’ed in. This being a first release, my initial password did not work but “alpine” still did. Since the BSD Subsystem was still not yet in place, I returned to my iPhone, launched Installer and got that and the Erica Utilities installed.

From there, it wasn’t hard to simply start using the iPhone the way I always and normally do. The version I tested did not yet have SpringBoard scrolling but that will be added before the GUI goes live. The one downside: some of the Installer.app items will not install because they “require Firmware Version 1.1.1″.”