This is going to happen, and perhaps sooner that folks think. But I think a systemic economic crash (imploding hedge funds, unwinding derivatives, bank transfer cascading cross-defaults) will be the proximate cause. IMHO the most likely trigger for this sytemic crash will be a middle east crisis, resulting in the closure of the Hormuz Strait to petroleum shipments, but there could be other triggers as well.
When that "just-in-time global conveyer belt" grinds to a halt, even if for a few weeks, our cities will literally explode. We have no surplus inventory, no way to ride out the disruption. One week after our ATMs, gas stations and grocery stores are emptied out from panic, our cities will be in flames.
It will be damned hard to just start up the machine again after that. An analogy is strangulation. If you choke someone for 7 minutes, you don't just let go and say, "You can get up now! Resume your heartbeat and breathing now, please!"
For example: once the gas stations have been drained to the last drop, and real panic sets in, a laden gasoline tanker truck will be worth a king's ransom. Fuel trucks will not be able to make routine deliveries. Criminal gangs will hijack them enroute, their cargo will be worth a fortune to anyone who controls it. A gasoline tanker will be an unguarded Fort Knox on wheels to anyone who can take it. There will not be enough police and national guard to protect every shipment of food and fuel.
This is the dark downside of our total global interconnectedness, based on JIT delivery. Shut that global conveyer belt down for any period of time.....and it's like shutting off the HVAC system in a modern skyscraper. The results will be swift.
Buy gold.
Some of my other friends point to China. China has been purchasing a significant chunk of our debt instruments, and just one wrong move with them and they will foreclose. H-E-double-toothpicks breaking loose then.