I just jailbroke my old Kindle 4 for fun. Found out of it ever connects to WiFi it unjailbrakes itself. :)
The email Amazon sent out said that if you factory reset your device after May 20 it becomes inoperable. I wonder if that means bricked, or if it just means you can't access your DRM kindle library.
I think you can disable the Over the Air Updates.
You will still be able to use it if you factory reset, but you won't be able to register it to an Amazon account or download any of your DRM'd book purchases. The Kindle will still work and you'll still be able to read books you load over USB. The one annoyance is there's a nag pop-up telling you to register your Kindle, but it only shows up in the main menu and not when you're in a book.