Update: The new jailbreak for 3.1 has already been released, so you can disregard the warning below. (Thanks to Jordi for the tip!)
Original post: If you wanted to use custom fonts or screensaver images on your Kindle but haven’t yet applied the jailbreak hack, make sure you do it before you update to the latest Kindle OS.
But hurry: Amazon is now pushing the new 3.1 software update over the air to all customers, and in fact you may have already received it and never noticed the change. (Open the “Settings” menu and check the bottom of the screen.) It brings several new features like page numbers and a more efficient magazine layout. Unfortunately, it also blocks the method in which the jailbreak hack was applied, as this MobileRead forums post explains:
It used to look for any public key which meant it would use the one added by the jailbreak. They’ve now altered it so that it specifically only checks the two Amazon Kindle public keys. This means all hacks will NOT be able to be installed.
The good news is if you already applied the hacks, they’ll continue to work. My custom screens are still working fine and I don’t even know when the 3.1 update was applied to my Kindle — I discovered it just a few moments ago when I checked my Settings screen.
The bad news is if you wanted to apply the hack but Amazon already updated your Kindle, you’ll have to wait until someone finds a new workaround — but fortunately someone is already working on it. Here’s hoping this setback is only temporary.