To get Xcode 4.2 on Snow Leopard to run code on a device running 5.1 you can do this:
If you have another Mac running Lion + XCode 4.3.1 you can copy the files from:
"/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport/5.1 (9B176)"
to the equivalent place on your SL Mac: probably "/Developer/Platforms/iPhoneOS.platform/DeviceSupport"
and also copy 'version.plist' from the Lion machine in the iPhoneOS.platform folder to the SL machine.
Re-start XCode on the SL machine and re-connect the devices and it seems happy enough.
[Edit: If you don't have access to a machine with Lion+4.3.1 you can get the files out of the 4.3.1 DMG which can be downloaded from Apple here:http://adcdownload.apple.com/Developer_Tools/xcode_4.3.1_for_lion/xcode_4.3.1_for_lion.dmg
then mount the DMG, Show Package Contents on the XCode icon and drill down to
/Volumes/Xcode/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport to
find the files mentioned above.]
转http://stackoverflow.com/questions/9614918/xcode-4-2-snow-leopard-and-ios-5-1