It depends a lot on the voltages and temperatures involved. If you're really having to up the voltages it might reduce the lifespan. However, if you're keen on hardware and overclocking, I doubt you'd notice the difference as you'd have upgraded well before any ill effects might come to light. I've really not heard of anyone having any chip suddenly die a death after a few years of overclocking. As long as its stable I think you should be fine. Make sure to run plenty of stability tests such as primt 95 and super pi to make sure.
Seriously though, I think you'd probably be reducing the lifespan by 5 years in 20 or something silly like that. It would be obsolete by the time anything bad happened.
