As mentioned, voltage increases do a lot more damage to a cpu than clock increases. I've overclocked everything I've ever owned, too, but I didn't start bumping voltages until june 07. I killed a cheap amd mobo that way, but the cpu still worked fine. Since then I've used better mobos and jacked up the voltages without issue. Hell, I ran an e6750 at 1.64v in bios for a few weeks until my room got too hot... I ended up running that one 24/7 at 1.53 in bios, 1.48 load. I sold it on fs/ft a year later and haven't heard any complaints from the new buyer. I told him the safe overclocking limits and my 24/7 settings, too, mainly because I thought they were slightly poor for an e6750 and wanted him to know.
I did a LOT of research before going so crazy on my e6750, then did more when I got a q6600, q9450, and x3350. The general consensus among knowledgeable oc'ers is that a typical cpu run at stock should last about 10 years. A mild oc with no voltage increase typical causes little to no shortening of life span. A moderate voltage increase, with or without an oc, will hurt a little more (lop off maybe a year or two) but still probably outlast just about every other component on the computer. A very aggressive oc with very good air or water will typically cut the cpu life in half. think q6600 at 3.6 here. A suicidal run with crazy good cooling will only last a few months (aigomorla has a good example or two of this). Poor cooling will cause a decrease in cpu lifespan, especially under high usage/oc scenarios, but not nearly as much as huge voltage increases. I believe that idontcare had a q6700 at 3.73 for quite a while on a vapochill, but he really needed that much performance and he only needed that setup to last until something better came out in a year or two. I don't think that he ever had cpu issues, but hopefully he'll chime in when he sees this.