An Ethical Dilemma...

Many of the problems and principles of introductory ethics deal with runaway trolleys and fat men. Confused! For those who are unfamiliar with these problems, let me elaborate. Here are two scenarios where a bystander can act or not act in an ethical or unethical manner (you decide):

Basic Trolley Problem:
A runaway trolley is hurtling out of control down a track toward five innocent people. If left unchecked, it will continue on its path and kill the five innocent people. Between the trolley and the five victims, there is a switch. If you throw the switch, the trolley will be diverted onto another track and kill only one innocent person. The question is...... Is it ethical (morally permissible) to divert the trolley, saving the five and killing the one?

Fat Man Problem:
Same out of control trolley, same five innocent victims, but this time there is no switch. Instead there is a fat man standing on a bridge overlooking the track. The fat man's weight would be enough to stop the trolley from hitting the five. Is it morally permissible to push the fat man in front of the trolley, stopping it, and saving the five?

Most people would say that in example one (Basic Trolley Problem) it would be permissible, or even mandatory to pull the switch and divert the trolley, whereas it would be impermissible to save the five in the fat man example. Both problems involve sacrificing the life of one in order to save five. What's the difference? Discuss!