iOS 5 SDK is gone after upgrade to Xcode 4.5
Current Xcode versions come only with the latest iOS SDK (which is a pity, but we cannot change it). Also Apple recommends to always use the latest SDK for building apps, and setting the "Deployment Target" to the minimum version that you plan to support.
You can copy an SDK (e.g. iPhoneOS5.1.sdk) from an older Xcode to
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKsand it will appear in the build settings. But you might run into problems later with this configuration.