Not all Kindles can be jailbroken. It's a constant cat and mouse game, and if you get a Kindle with up-to-date firmware, there's a good chance you cannot jailbreak it for now. Once jailbroken, you need to make sure it doesn't auto-update the firmware. And personally, I think one should rather support open devices. Even if Amazon loses money on a jailbroken Kindle (which I doubt), in the end, it still hurts open alternatives.