What reduces the life of components are heat and overvoltage. As long as the heat is kept down and voltages do not go above specifications, you won't shorten the lifespan. Even if it didto a slight degree, by the time that came into play you would have long moved on to another cpu. So, depending on the system, a mild to moderate overclock will not harm it. It's the extreme clocks that get dangerous. If you plan on doing any, research first, there's a lot of good reading out there. Blindly flailing away at your bios isn't a good plan. If you haven't already, read the sticky.
http://www.techsupportforum.com/hard...-starters.html