They fall off frequently enough that the condition, if they're lucky enough to survive, is called "high-rise syndrome." On the cat's medical record it would read "HRS" and they would typically arrive at the vet's emergency room with front legs broken from being jammed into the ground, broken jaws and facial bones, and air in the chest cavity, for starters. Cats who fall from higher floors have a better survival rate because they have time to "right" themselves and make a less injurious landing.

http://www.aspca.org/site/PageServer?pagename=pets_highrise