Interesting, I found this tidbit at VeloNews:

Wayne Stetina, Shimano's R&D manager, says, "If you remove the chain when it is only halfway worn out and flip it over," he says, "you will double your chain life." In other words, your chain will now be turned inside out. The other side of the rollers will now contact the gears, and the derailleurs will now be laterally bending the chain the opposite direction. Stetina says that Shimano engineers discovered this phenomenon quite by accident.