Hi, I have managed to persuade my Kindle 3 to load and run a (very simple) kindlet I've written. I do not have access to the KDK: I am using jars I pulled off the kindle itself to develop against.
The trickiest bit was figuring out how to sign a "developer" kindlet properly.
There are details and a demo kindlet source and binary here: http://adq.livejournal.com
BTW: thanks for the jailbreak and usbnetwork patches; they're really cleanly done, and saved me loads of time!