The highest jump ever made was by Joseph Kittinger in 1960--31 km. This year, astronaut Felix Baumgartner will attempt a 36 km jump, and there is talk about Michael Fournier attempting a 40 km jump. According to medical director Jonathan Clark (via Science magazine, subscription required), jumps from this height have a variety of health risks, including nitrogen bubbling from the blood, sweat freezing on the skin, and spinning while falling -- leading to a brain hemorage. There's also the issue of what will happen when he breaks the sound barrier, which could produce massive shock-waves (though there is precedent for a test pilot surviving after his plane broke up while traveling faster than the speed of sound).
The reward? Aside from glory, the data collected is expected to give us a better idea of the risks involved in the next generation of manned space flight.
Good luck to these daredevils.
