Out of five i7 920's, I've had one fail due to IMC failure early on. I believe this was primarily due to heat and possibly the VTT was set too high. The other processors have been running strong since early 2009. I keep these processors loaded 24/7, clocked between 3.8-4.3 GHz depending on ambient temperature, and VCore set up to 1.45v. I keep the core temps below 80C. I have the VTT set as low as possible these days and I avoid excessive Uncore frequency to save the life of the IMC. In the case of the 920, that basically means keeping the memory frequency set at 2:8 with higher bclk and generally the VTT can be kept near stock voltage.

When i got my i7-950 i was new to overclocking. I oveclocked it afer the first couple of days fully knowing that with my level of expeirince i could fry it right off the bat. Fortunately nothing has happened yet and im sure having a high quality motherboard like mine helps. Right now ive got my 950 running at a 3.7Ghz clock at 100% load 24/7. Thats pretty rough stuff but we will see how long it lasts for. Ive had it folding for EVGA for about 6 months now. If it does die the 950 has come down a bit in price since i bought mine. Or maybe it will just be an excuse to upgrade to socket 2011, even though i cant really afford it.

I think that most overclockers get into it knowing that they might fry something. So for this reason i dont think that CPU lifespan is an issue to anyone unless your overclocking because you need the speed for something like intensive rendering or encoding. 

Computers are like cars, you can buy the cheap Toyota that will last you the rest of your life, OR you can soup-up that BMW and get a few years out of the engine.

The start that burns twice as bright lasts half as long.