A quick search on Xeon production yields that it goes through a rather rigorous testing. I wouldn't be surprised that server cpu's in a desktop pc works longer. I can't overclock it either, and that probably helps with its lifespan as well. But yeah, the fact that it actually powers on when i click the button and isn't a limiting factor after 10 years is quite something.
Back from my old overclocking days - its heat that kills life. And if you keep that under control (what ages is the heatpaste, replace it ever so often) i very much doubt you'll have any life issues from the cpu itself.
Bearings in fans, caps etc. are also stuff that you need to keep an eye on.
I just replaced a i5-660 thats been powered on since 2010 24/7, heatpaste was fucked so it crashed during heavy loads :)
