I define a "SHTF" scenario as one which presents an immediate risk of death or serious harm to you, your loved ones, or even strangers around you.
The SHTF when the badguy sticks a gun in a clerk's face.
The SHTF when an officer has to fire his weapon.
The SHTF when a CCW holder has to draw his weapon.
The SHTF when somebody breaks into your house with you there.
The SHTF when a natural disaster hits where you are.
Note: The blue helmets wandering around in the USA would be a TEOTWAWKI (The End Of The World As We Know It). Nuclear attack(WMD), Super disease, Extraterrestrials, Zombies, Mutant Ninja Zombie Bears, and such are these scenarios.
The Tsunami that recently hit was a SHTF situation for the people in the area. It was a TEOTWAWKI for the survivors.
I feel prepared for many SHTF scenarios. TEOTWAWKI, well, I figure that I'll be reporting to work, as best as I can, and not have a personal life for a few years, or die trying to prevent it's spread. Work will provide the food, water, and clothing under most scenarios, at least for a few years.
*edit: lee n. field, Ky Larry, those are TEOTWAWKI scenarios, they're SHTF for the entire world. A SHTF is generally considered more of a personal or localized event.