I suppose I am justified of adding additional comments to this thread, as I have spare time currently.
Logically, you can not just simply state a maximum
value for a particular model of processor. You can give a maximum recommended value, given the era.
Each processor contains slight variations at the design level that must be taken into account when defining an acceptable thermal, or power, level of a Micro Processor.
How we relate to this difference is the Week of Silicon that is used within the manufacture of these Micro Processors. The purity of the Silicon (which is a major element used in the construction of Micro Processors) is vital in the assumption.
In truth we quote a value which we recommend
as the maximum voltage that the chip can sustain for an acceptable lifespan (this is approximately five years).
However, even if the chip was supplied with a voltage of 0.40v (Core 2 Duo with default operating voltage of 1.3250v) it would not last forever.
No matter what voltage value that is used the chip will still degrade, it can never be stopped in reality, just postponed with an extremely low voltage to as much as several hundred years perhaps.
MjrTom is partially correct with his assumption that Quantum Tunnelling is the effect that renders the Micro Processors un-operational after a certain period of time. However, this issue is far more related to the flow of electrons (Current).
Going into "low end" Physics we have a selection of basic formulas that are obeyed in our reality, and dimension (exception is past the Schwartz Radius, black hole [singularity]).
Given a constant temperature, or very similar temperature the Resistance will only suffer a minor change, as Resistance is affected by a change in thermal levels within a system (major factor).
When Voltage is increased it is logical to state that the Current (I) will also change.
When overclocking it is normal to increase the voltage, therefore increasing the Current. This is the major factor in the ability of Quantum Tunnelling.
Therefore, Voltage and Thermal Levels (Related to Resistance) are indeed important in the ability of a Micro processor to sustain itself.
I will not give you a maximum recommended value for the Core 2 Duo series, as the recommended value has changed due to revisions in the chip itself, including the increased purity of the Silicon used in its manufacturing.
As a comparison my E6600 (week 28) is capable of operating with a voltage of 1.625v for a lifetime of five years or greater without sustaining major damage, I do not know it's ability to sustain itself after that time frame (when my exams have been completed I will run you through how to estimate
The Life expectancy could be modelled by an exponential graph. Perhaps using an equation similar to the discharging of a capacitor (or equivalent) [V=Vo.e^(-t/RC)]
To thread starter: The choice is yours, if you have an early Core 2 Duo processor the voltage you have chosen (1.5v) is acceptable for a life expectancy of five years or greater, according to the manufacturer (not officially stated). With later processors you must be wary with the voltage selected.
Note: Silicon is capable of "multi oxidation states", and therefore is able to undergo more advanced electron transfer than elements with single or di-oxidation states (or equivalent). This is not in direct relation to the points expressed above.
I am currently studying for my final Chemistry exam and have had my Engineering Books removed from my room, therefore I can not go into extreme depth on the full effects of additional voltage, Quantum Tunnelling, and equivalent