I have a mid-2010 MBP with Mountain Lion. I performed a format and re-install (2 times). The first and only thing I installed was XCode (most recent version is 4.5.1). I have tried to run some of my projects, also just a plain blank project fresh after the create menu (all target iOS 6). Compilation is fine, but when I try to run on iPad/iPhone simulator, as soon as the iOS simulator is launching the screen goes black and the system restarts. Any ideas on how I could fix this? From Googling I've not seen anyone with quite the same issue, but have tried things such as deleting plist files, changing the debugger used, using no debugger, but it still crashes.
It turned out to be a fault with the graphics card. I am getting it replaced.