The transistors may be damaged by overheating to the extent that the chip doesn't overclock as high as it would've but, if you aren't trying to hit the high notes you may not notice anything in real world performance terms. Check out
this vid. from jayz 2 cents at about 3.00 he somehow overheated it and found that it's no longer an amazing overclocker.
What's the takeaway? Always strive to never overheat the cpu even once. It would probably take a sustained overheating to kill it completely but, chance being what it is, something important could die the first time but it would be more likely to die if you were running it too warm over time with a cooler that had a failing pump or fan, or the thermal paste on it was going off, and particularly on warm days when ambient temps are up and idle temps are higher, those are the sort of conditions where cpus can die. Besides over-volting which can occasionally happen as well if there is some electronic fault on a board.