Xcode 4 has an awesome feature called implicit dependency finding (in the Edit Schemes sheet). That wreaks havoc, but only at some time.
To allow Omni frameworks — namely, faux iOS frameworks — to build reliably and correctly, add the uninstalled products directory to the user header search paths…
Very, very useful. Building Omni frameworks in a plain Xcode project can be something of a serious pain in the arse otherwise…