I face the same thing when I installed the XCode4.5. Then I copied the SDK from my old xcode to the new xcode. The SDK is located in the following path.
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk
/Applications/Xcode4.5.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk