[HorseSoldier]

Likewise the mythical "urban/suburban surge" where the city folk descend on the countryside and warfare to the death ensues. These are taken as articles of absolute faith by survivalists and the post-apoc genre in general, but I have my doubts it would go down much like that.

It isn't really debatable that when you pull the technology rug out from under modern societies and their agricultural and distribution systems that major urban areas become untenable. So those folks have to go somewhere.

I agree that this likely would not start as ravening hordes pouring out of urban areas, though it might end that way. It would likely start as shown in T2K, with an attempt to move urban populations closer to the means of agricultural production. Unfortunately, with the petroleum inputs modern agriculture depends on stripped out of the equation, agricultural productivity will plummet to the point that just supporting the local population will be an uncertain proposition.

And that likely plays out well before someplace like Florida has been able to fully empty its urban areas to sustainable levels . . . and so a second wave of hungry and desperate urbanites heading out into the countryside is a definite possibility.

Thankfully, casual cannibalism simply doesn't happen in the real world, even in the face of famine.

Casual is a bit of hyperbole, but cannibalism certainly does happen in the face of famine and starvation. If you add in a base population that lacks any significant awareness of any sort of direct subsistence strategies, like most urban populations in modern societies (or put another way, the bulk of the population in all modern societies), and also the utter collapse of established social structures you've got conditions that would lead to widespread cannibalism by desperate people. A lot of places in post-nuke America and Europe would be somewhere between "20th century communist agrarian reforms" and "stranded arctic expedition," and cannibalism -- most likely primarily opportunistic scavenging of those already dead and less common active predation -- would certainly be occurring.