Um, actually...you hardware (well, I don't know about YOUR hardware but...) has an actual life expectancy of something like 1,000,000 hours =) Thats roughly 114 years incase you're wondering. Overclocking takes away a few thousand hours of your life expectancy. at most it'll take off 1% (10,000hours).
In the end, the verdict is the same, overclock all you want, don't worry about life expectancy. But the thing you SHOULD worry about is overly high temps. If your temps are too high (over 55*C) then your hardware can start to malfuntion. I mean, random freezing, rebooting. If your temps get overly high 75*C+, then your hardware will start to fail immidiately. Sometimes the processor will simply burn out.
But for most people, high temps isn't something they have to worry about with reasonable overclocks and stock cooling.
