Well, as was mentioned in a comment above you can get the root shell pretty easily. With the upgrade package format deciphered , you can evidently "upgrade" your kindle to e.g. run anything at the startup time. I made mine look for a script in a specific path in the user-mountable partition and execute it if found. From there on, no special tricks were needed. With dropbear available precompiled for the arm arch, and after executing one of the "debug" commands to switch the USB port to the Ethernet mode  you get the shell access. Direct modifications to the system partition are possible; just mount -o remount,rw / first.
My motivation for the hacking was a perversive desire to read e-books in other but the English languages. Come on, no support for Unicode in the 21th century?! It turned out the Unicode support is there (well, except for the bidi languages), it's just the fonts that were intentionally castrated by the kindle developers. AFAICT this is mentioned neither in the docs nor in the FAQ. Apparently, Amazon believes Americans never read in a foreign lingua.
On the positive side, there is a rather large built-in Oxford dictionary; as someone for whom English is not a mother tongue, I find it indispensable.