The question is this: What is the impact of running the CPU at 100%
for long periods at a time? Does this degrade the life of the CPU or
other components like overclocking can/does?
To answer your question, yes.. running a cpu at full capacity for extended periods will shorten it's lifespan. A combination of heat and flow of electricity through the chip causes changes to the components within the chip at the atomic level, known as electromigration. Here's a brief (and very simplified) explanation:
This is when metal atoms wander into the dividing layers on a
microprocessor. It is caused by the combination of electricity and
heat. Processors are designed to run within certain heat and
electrical specifications, and if run at higher heat and/or electrical
specifications, electromigration may occur. If this occurs to a great
degree and enough metal atoms wander off of the lines in a processor,
they may permanently ruin the processor by thinning a connection so
that it does not work effectively, or even making an electrical
connection where one is not intended to be. Overclocking and raising
voltage supplied to a processor increases the risk of
A more in depth explanation can be found here: http://www.csl.mete.metu.edu.tr/Electromigration/emig.htm
So taking this information into consideration we can deduce that more heat and more volts cause an increased level of degradation due to electromigration. Running the CPU at 100% means it will receive more volts and therefore more heat (for sustained periods), ultimately contributing to the shortened lifespan (literally speaking, without taking into account how long and how often these periods of activity occur). Of course, the point is almost moot because as you state, your temperatures are good and well within thermal limits so it would take a long time for the processor to degrade to the point of failure - i'd be more concerned about the electricity bill!
With regards to overclocking, to attain higher clocks - more volts are required at the expense of increased thermal output. Taking into consideration the information above, pushing more volts through a processor will cause the chip to degrade a lot faster (timescale obviously depends on the amount of volts and how the processor is used).
Hope this information helps answer your question..