The Nook Tablet hardware protection was hacked with some help from the Bus Pirate. The Bus Pirate was used as a simple USB to UART bridge so nook hackers can talk directly to onboard ICs and bypass the standard bootloader.
The device was released allowing ADB to be accessed. Barnes and Nobel quickly patched this hole. o XDA Recognized Developer AdamOutler released a reversion package imediately, and XDA Senior Member Dean Gibson worked on restoring this ability in 1.4.1. AdamOutler then released a Nook Tablet HackPack, discovered a method to boot the Nook Tablet securely from SDCard and showed us how to set up UART with the assistance of XDA Senior Member Pokey9000.
Finally, pulling together all the techniques learned, member hkvc was able to totally bypass the device’s security at the bootloader. He calls it NOP Based Security Bypass. hkvc’s technique loads a custom kernel into memory, modifies security registers, then executes a custom kernel. This requires a hardware modification and may lead to the first mobile device with a modchip. However, using the power of booting from SDCard this will be an unpatchable security exploit and allow custom ROMs like CyanogenMod and Ice Cream Sandwich to run on the Nook Tablet eventually.
Via the forum.