My unsanctioned opinion: Only proceed with this downgrade if you are willing to risk bricking your Kindle. If you are comfortable opening it up and soldering on a serial port in the worst case, this is probably a good method for you. I'd wait a few days or weeks for more adventurous and technical members to post feedback so we can get a better feel for if this is dangerous or not.
(this is much riskier than simply exploiting a process, i.e. the previous JB.) I'm guessing if we had contact with Amazon they would not recommend this technique.
