When discussing overclocking CPU's you need to take into account both Voltage and Temperature, they cannot be treated as separate parameters, they go hand in hand.
The main reason CPU's fail is due to electron migration.
Electron migration occurs when the flow of electrons through the CPU core architecture "jump gates" or erode the traces causing failure of the pathway or gate. This permanently disables this section of the cpu.
The main cause of this is elevated core voltage. As the voltage is increased the electron potential increasing the energy at the same time.
Temperature on the other hand plays a lesser role in CPU failure as Intel CPU have had "throttling" measures inbuilt into the architecture to prevent overheating.
For the older members of the forum, some of you will remember the older intel CPU's in particular the Northwoods.
These suffered from a recognised problem known as Sudden Northwood Death Syndrome (SNDS) whereby the CPU would have good load temperatures but at a certain voltage the CPU would just die, no slow decline..... Poof... Dead!
Im not saying that this would happen with the newer C2D processors but the electrical engineering principles are the same.
Remember its BOTH voltage and Temperature that shorten the lifespan of a CPU