These are the steps I took to unlock the latest OTA. They worked for me, they may not work for you. Anything you do to your phone is not my responsibility.
- Backup what you can. Your phone is going to be wiped.
- Make sure your battery is charged up.
- Download the full 1.55.531.3 image. This contains the confirmation splash screen which wasn't loaded on older MT4GSes, and is the reason the unlocker tool won't work on our devices. You can get the full image (it's over 400MB) here:
- Download the CWM recovery image:
- Rename the full image downloaded in the first step to PG59IMG.zip and place it on the root of the SD card in your phone.
- Shut the phone down, remove the battery, and boot back up while holding both the volume down and power buttons down.
- The phone will check the PG59IMG.zip file and will ask you if you want to restore that image. You want to do this, but keep in mind this is going to wipe your phone. All data will be lost.
- The phone will restore the image. This takes a few minutes. When it's done, you'll be prompted to reboot, and you'll boot into a completely stock MT4GS experience.
- Use the unlock tool on htcdev.com to generate your unlock code. The splash screen is now on your phone, and you should be able to unlock (I had no issues).
- I rebooted at this point, just to be safe, and set up my phone.
- Copy the recovery image you downloaded earlier into the directory where you're running fastboot. Rename it to recovery.img
- Shut down, boot into the bootloader/fastboot, and enter the following command to flash CWM recovery:
fastboot flash recovery recovery.img
- At this point, you should probably reboot the bootloader before entering CWM. Once you've done that, you can go ahead and flash superuser/roms/etc.
Here's a pointer to restoring your phone to stock, courtesy of blue: