In fact Most of the ui is running in java.
And Amazon is sending beta for the Kindle dev kit
https://kdk.amazon.com/gp/vendor/sig...s%2Fkdk%2Fhome
also see
https://kdk.amazon.com/gp/vendor/kin...t?id=200436000
but it is java 1.4me
My take is that java 6 is quite possible, using the openjdk and compiling it for arm6. But thinking of the effort it requires I would start using the existing jre, as I think that using 1.4 would not be that limiting.
If you want to start this way there is
http://adq.livejournal.com/107360.html which explain a good deal about developing kindle java programs.