XCode 4.4 iOS 5.1 Simulator problems
As a workaround, I've noticed that Running (⌘ + R) and then Bulding (⌘ + B) product succesfully starts new simulator instance, although the old instances are still frozen.
I just reverted to doing something I used to do when this started happening to me back in XCode 3.1 or something.
Press the STOP button in XCODE before you close or background the app in iOS SIMULATOR.
Then close the iOS SIMULATOR.