A friend of mine is having an issue, and I can get more info as needed.
Basically, during times of stress on wow (heavy pvp, heavy raids, large population) the game will just close. Not disconnect, not crash (at least no error report or 'send to blizzard' report). It just... closes, as if Alt+F4'd
He has since wiped and reinstalled Win7 64bit (I don't know if he's using WoW 64bit), updated all drivers, and is still having the same issue. He's using a liquid cooled system and states heat isn't an issue (however I have not had him check actual temperatures. I'm having him use hwmonitor to check temps and he'll get back to me next time it happens)
CPU: AMD Phenom II X4 955 Black
RAM: 12GB DDR3 1600
GPU: ATI 6950
As I write this, he's on vent saying it just closed while him and a friend were fighting a rare mob, so not a populated area. He said that's the first time it's closed outside of a raid or battleground.