no one knows what the lifespan of these chips will be, at stock or overclocked. All the above are guesses. What can be said for sure is that processors are run hard, and rarely die. There are 4ghz q6600s which were clocked years ago and haven't shown signs of dying yet.
However this might not extrapolate. Now we're on 45nm, looking towards 32nm. Speeds and power consumption are up significantly, so elctromigration is going to become more of a factor as time goes on. Eventually the '20 years down to 10' is going to be a fond memory. However I doubt we've reached this point yet. 45nm quads have been thrashed and kept running for several years.
So I join the above in saying, effectively, "who cares"? It'll be good for the usable lifespan of the computer anyway. If however you run at 100 degrees, 1.5V, it will die and it will be your fault for not researching it. Keep it cold and keep the voltage somewhere sensible (within the Intel vid range counts, if you're running at a lower voltage than intel ship processors at it can't be too dangerous), and you'll be fine.
Lifespan of a processor. So the hotter it is, the faster it will die, but equally the higher the current density, the faster it will die. So if you run hot with high current density, it's not going to do very well. Either one of the other tends to be fine, high voltage and cold or low voltage and hot.
