The Nook Tablet has become notoriously difficult to mod, thanks in no small part to Barnes & Noble's decision to ship it with a locked bootloader. Various software updates have closed root methods already in use, making it more and more difficult for modders to do what they wish with they hardware they bought. Thankfully, XDA member Indirect has combined a series of hacks to create a root method that launches directly from a MicroSD card, which boots directly into ClockworkMod Recovery.
The process brings back memories of the original Nook Color, which was a breeze to modify thanks to relatively open software and a boot priority that looked to the SD card storage first. All you have to do to get the custom recovery files to boot is copy the files provided to a card, insert it into the tablet and reboot. (You don't even have to format the card, unlike some Nook Color methods.) Once inside ClockworkMod Recovery, you can flash custom software or use another hack to get ADB file pushing from your computer. Flash the root ZIP file and you're done.
This method works on Nook Tablet software versions 1.4.1 and previous, according to the XDA post. IT doesn't do anything about the locked bootloader, so basic modifications of the stock software are still the best you're going to get on the Nook Tablet, while the Nook Color enjoys ports and ROMs galore. Recent mod chip-style hardware modifications may lead to a breakthorough on the bootloader front, but that will require time, skill and - the tough one - patience.