Just in case anyone else ever gets stuck in that situation, here's something to maybe make you feel a TINY bit less afraid.
Most elevators have redundant safety systems to prevent you from falling very far before safety brakes engage. But even if all systems failed simultaneously, there's almost always a shock absorption system at the bottom. for instance, there might be long metal pistons that plunge into a very viscous fluid to bring the car to a halt quickly. It would be abrupt, but would probably prevent serious injuries. There are different last-resort features that vary by manufacturer and model. plus - as an elevator car approaches the bottom of its shaft it will compress the air in the shaft to create a natural cushion.
Of course, I would probably die of a heart attack before the shock absorbers kicked in.
Here are some fun related links I found. sorry they have to be decoded, but i can't post URL's yet. 
Answers about falling elevators. Does it help to jump up at the last minute? what's the best position to fall in? and more...
van (dot) physics (dot) uiuc (dot) edu/qa/listing (dot) php?id=139
info on those safety features i mentioned:
express (dot) howstuffworks (dot) com/runaway-elevator (dot) htm
info from worst-case-scenario aka popular mechanics:
www (dot) popularmechanics (dot) com/science/worst_case_scenarios/1289266 (dot) html