Thanks for the hint eureka, unfortunately Kindle 3 & 4 doesn't have /etc/todo/handler_registry.conf nor otaupd daemon.
On Kindle 3 & 4, Transfer Manager Daemon (tmd) handles the downloading part, reports back to framework via DBus when the download is complete and then the framework creates file /tmp/.framework_update (containing path to the downloaded update file) and, on the occasion of entering sleep mode, launches process_update which in turn launches updatewait which in turn launches process_update -i after some preconditions are met (battery level sufficient etc.).
"Update Your Kindle" launches updatewait -i which in turn launches process_update -i.
Last edited by dsmid; 02-14-2013 at 03:17 PM.