I bought a Kindle a couple of weeks ago. I think I had a jailbreak installed within two hours...
Alas, the security policies mean that we can't actually get out of the JVM sandbox (including creating our own class loader to make Rhino's JIT work, curses curses), but it's still quite handy.
Don't suppose anyone's figured out how to do custom fonts in a Kindlet? The standard monospace font looks like suck with antialiasing turned off (needed to make it comfortable to type on).
People may be interested to pull it apart; among other things, it's using retroweaver to let me use Java 1.5 constructs on the Kindle.
There's a binary too (which includes installers for my public keys so people can run it).
I've also written up some notes on how to get started with Kindle application development:
I hope this is useful to someone...