Well, you should have installed it before
you upgraded to 2.5. I don't remember exactly how it was implemented, but if it basically added a shell script to be run at startup time (an rc script), the upgrade shouldn't have removed it. While performing the upgrade, Amazon's updater checks the md5 hashes of files in its long list, and it leaves the other files alone.
It seems that Amazon disabled JVA's jailbreak hack, which installed temporary public key before installing the hacks. I guess, they used to remount rootfs for read-write before they untar'ed the update packages, but now they switched: first untar, then remount. This probably means that the jailbreak may not be possible for 2.5.
However, if you had means of executing a custom script remaining
on your device from 2.3, you should be okay. This script will let you make your own jailbreaks, dumps, install fonts, USB networking, etc. In other words, if your Kindle had been "rooted" before, it can stay "rooted" after 2.5 upgrade, but you cannot "root" your device anew after you have installed 2.5 on it.
I was looking at the Chinese forum yesterday, where they reported
that the "unicode hack" (Chinese style) still works in 2.5. What it really means is that after upgrade to 2.5 their script /etc/rcS.d/S73linkfonts is still in place.
Now, if anyone happens to know of an update that will take us back from 2.5 to some previous version of the firmware (downgrade), that would be of a real value...