This does make me wonder what is so special or unusual about your various Debian installs. Outside of attempting a woody->sid dist-upgrade in the distant past,

This does make me wonder what is so special or unusual about your various Debian installs. Outside of attempting a woody->sid dist-upgrade in the distant past, I have never had problems with "apt-get [dist-]upgrade" breaking anything to the point of not booting, and certainly not a simple "apt-get update" to refresh the package list. (This is using both Ubuntu and Debian stable.)