To enable iOS 5 support in Xcode 4.1, read on.
You've upgraded one or more devices to iOS 5.0 and try to use Xcode 4.1, only to be told it doesn't recognize the version of the OS. Why would you care? Apple has been advising for some time that gcc is going away, and that date may be fast approaching. Unfortunately, Clang / llvm lack some features available in gcc, such as register assigned local or global variables. We've only just gained access to a version of Clang that supports C++, and so you may still need some time to migrate your code.
Assuming Xcode 4.1 is in /Developer and Xcode 4.2 is in /Xcode42, run the following, substituting the appropriate iOS 5 build numbers:
cd /Developer/Platforms/iPhoneOS.platform/DeviceSupport ln -s /Xcode42/Platforms/iPhoneOS.platform/DeviceSupport/5.0\ \(9A5313e\)/ 5.0\ \(9A5213e\)