70's fairly high, but if its stable, and it doesnt throttle its fine. Coretemp/Tat/etc are all well and good, but the most important number is often not displayed. The delta. As long as the delta doesnt reach 0, then the chips not at its maximum temperature. At 70 degrees you could have anywhere between 15 and 35 degrees "spare" before you hit the throttle point, depending on the max tjunct of the CPU.
Ok, not modern 45nm tech, but oldschool 80486! on a small passive heatsink at 33mhz, and it used to hit almost 100 degrees as fairly "normal" temperature. Geforce 6800's didnt throttle until 120 degrees!. Chips can get pretty hot before they get damaged, and all Intel chips since P3 have had the ability to shutdown once the core gets too hot, P4 and Core Duo's throttle more gracefully.
Of course overclocking does reduce the lifespan of a processor, and so does increased heat. But the odds are the chip will be obsolete before it breaks. Overclockers dont seem to stick with 1 processor for very long... its out with the old and in with something newer long before the chips are fried.