It has been reported previously that Amazon adds a few hardware-based features that can lock down the Kindle Fire HD pretty tightly and makes it harder for hackers to find an effective workaround. One thing that Amazon seeks to avoid is an access to Google Play store, which directly competes with Amazon’s applications catalogue. Apparently, developers and testers only need a few days to bypass the heavily-protected bootloader in Kindle Fire HD. Also, the same method was used to break into the Asus Transformer Prime.
Users implemented the exploit by downloading Super User and running a number of commands that allows them to get a root. A thread in XDA Developers forum provides a complete instruction should you have trouble along the way. For the time being, this procedure only works on the 7” version of the Kindle Fire HD and as time passes; we should hear confirmation of successful rooting attempts on other version.