Try this on the new Xcode 4.4: create a new project, a Cocoa application, and set the Development Target to OSX 10.5.
If you run it, you'll get this error:
Command /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang failed with exit code 1
Can someone explain this? I want to support Leopard too in my projects but I get this error if I try to execute them.