I recently decided to jailbreak my Kindle PW2, so I could use KOReader among other things. Unfortunately, it was running firmware 18.104.22.168.2. A serial port jailbreak is in order!
The cheapest USB to 1.8v serial adapter I could find was a Deek-Robot PL2303TA from Amazon for $4.98 and free shipping. It ships from China, so it may take awhile to arrive. Before
opening your kindle you may wish to copy down the serial number and extract the jailbreak to the root of the mounted filesystem.
To open the Kindle, pry off the recessed faceplate using a flat-head screwdriver or knife. Mine was glued but came off surprisingly well.
Next, remove the 11 screws with a #0 Phillips head screwdriver. The screw in the top middle is underneath adhesive tape.
Remove the plastic case and flip the PCB over. You may want to remove the battery before soldering. Solder the Tx, Rx, Gnd, and V+ to the correct spots. On my adapter Green is Tx, White is Rx, Black is Gnd, and Red is V+.
Now it is time to get Root! Follow this guide http://www.mobileread.com/forums/showthread.php?t=263113 for using minicom and applying the jailbreak.