Well, Dave, you and I have already discussed how ideal my apartment would be in the event of a zombie apocalypse.
I remember reading about when the USSR collapsed. People woke up one day and their nation's capital, Moscow, was suddenly in a foreign country. If something like that happened here, I don't think we'd be seeing riots in the streets of Hillsboro (however I wouldn't go near East St. Louis unless my life depended on it). I think people in small rural areas would band together pretty quick. Essentially, you'd end up with thousands of "mini-countries" dotting the countryside living under a localized form of martial law.
The cities would fall into chaos pretty quick. Think about it: no police, no fire department, no hospitals, no electricity, no running water, no phone or television service... nothing. People might band together initially, but eventually hunger will set in (because we're talking about city folk who don't know how to grow their own food). And when people realize that the government that they trusted to help them no longer exists, then the city's population will begin feeding upon itself. It will turn into a "might makes right" nightmare.
Just think of the LA race riots from 1992, then multiply that times the Hurricane Katrina disaster and you'd have that in every single city in the country with no hope of law enforcement or even a military putting a stop to any of it.
Of course, much of that would bleed off into surrounding towns, but I believe that most rural towns would seal themselves off from the outside world. Trespassers would be shot on sight. Eventually, localized militias would begin to form to restore order. They would be made up of just regular citizens with firearms who want to keep their families safe. Maybe led by former military and police, but not necessarily.
The most tragic part will be when the economic system collapses and the banks shut down. People's bank account balances will vanish into thin air and the credit cards and cash that they have in their pockets will become worthless. You IRA, retirement plans, health insurance would all disappear and people would be forced to barter for food and clean water.
After just a few days, stores, gas stations, and restaurants would be looted clean and whatever food was left behind would be rotted. People would start to eat their pets and their pets' food. The next source of food would be the garbage, people would be forced to rummage through dumpsters and city landfills for any food they might be able to find. Ranchers would be on almost 24 hour alert in order to prevent people from entering their land and killing their livestock for food. Also you can't rule out cannibalism, especially in the overcrowded cities.
I can't imagine what our borders would turn into, because if we go down, Mexico and Canada are going with us.