NOTE: This is only for unrevoked forever users.
Tested to work if flashed from hboot (bootloader)
Not tested in recovery.
NOTE 2: Make a nandroid backup.
Place file on root of sdcard, power off your phone, then power on your phone by holding volume+down and pressing+holding the power button until the white bootloader appears. Press volume up once asked to install the bootloader.
NOTE 3: For those experiencing being stuck in bootloader, you can boot into recovery, flash your current ROM again, (without data wiping) OR just restore a nandroid backup. Flashing a custom kernel will also get you out of the loop. I'm going to go ahead and assume flashing anything other than hboot iwll get you out of the loop.
This will NOT wipe your device, however, backing up is always recommended.
UPDATE: It appears as though everyone is getting stuck in bootloader after this procedure. You can get out of this by reflashing your current ROM (without wiping) and/or restore a nandroid backup. Some have reported errors when trying to restore a nandroid backup. YMMV
Karadorde has put together a package that works. Read his instructions below:
First and foremost it seems that if you get stuck in the Bootloader (no matter what method you are using) then your Nandroid backups are broken. I don't know that anyone has been able to successfully restore from one of those yet.
The file in the OP allows you to apply just the engineering HBOOT.img (hboot_0.76.2000.img) This works flawlessly for some but not so well for others.
The file in Post 71 and Post 91 allows you to apply a full PC36IMG.zip that contains the engineering HBOOT.img (hboot_0.76.2000.img) This is a fix for some who have gotten stuck in the Bootloader after using the file from the OP. At the same time this has also caused the some of the same issues people were having with getting stuck in the Bootloader from the OP. This file will not downgrade any part of your phone, however, it will wipe the phone. Also it is reported that it broke root on one phone. I believe this only happened to one person, need confirmation on if it happened to multiple.
If you get stuck in the Bootloader you can re-flash your Kernel to fix the issue. Not sure if this works with the stock Kernel or if you have to have a custom Kernel. Looking for feedback on this.
If you get stuck in the Bootloader reflash your current rom. Need feedback on this as well.
Link to Karadorde's updated package: http://www.freeza-inc.com/PC36IMG.zip