A malicious upgrade (in my mind) is one that is released very closely to a jailbreak under circumstances where you can reasonably infer that the prime purpose was to patch that jailbreak. I'm sure it will fix something else as well, they always do, but if the upgrade fixes a timezone change for a city of 800 in the Southwestern Outback and patches the jailbreak, I think it qualifies as a "malicious upgrade." I think people need to remember to disable their OTA upgrades.