I just got stranded on a freeway off-ramp. No warning, the car just stalled and wouldn't start. It acted like the timing belt had just snapped, but I changed that recently so that's not it. I noticed that there was some coolant that had sprayed around a bit in the engine compartment, and the temperature was pretty high - though not in the red. After the tow home ($$), I noticed that the top hose going to the radiator had slipped off. So that explains the coolant everywhere, but it doesn't explain the stall. Other than the temperature reading pretty hot, there were no symptoms of overheating.

What would have made car stall out completely like that? Is there some kind of fail-safe that shuts down the car if there's a significant coolant leak, or the temperature starts to get too high? My car is a 2001 Outback with the 4 cyl. engine.

The coolant drowned your coil pack and shorted the spark to ground. 

Spray it down with some WD-40 and then wipe off the excess. Pull the plug wires out and get inside too.

OK thanks, I'll try that.

What are the chances that my engine is toast? Would there be a lot of steam, and would my temperature gauge go into the red if the coolant were that low?

I'd say your chances are pretty good. The coolant dousing your spark would have happened pretty soon after the hose came off.

Even though, once all the coolant is gone, the temp sender won't show how hot the engine really is because it will only be sensing the air in the empty coolant passage. 

Good news, the engine just started up again, and sounded fine. Although the battery is obviously low on account of leaving the hazard lights on so long, so it wouldn't start up a second time. Looks like I lost about 3/4 gal. of coolant, so hopefully that wasn't enough to cause damage.