Project Builder complains about precompiled headers?!

If you get an error like "warning: could not use precompiled header '/System/Library/Frameworks/Carbon.framework/Headers/Carbon.p', because:
warning: 'OpenScripting/OpenScripting.h' has different date than in precomp", then very likely your precompiled headers are out of sync with the actual headers and the installed software. This can happen since some of Apple's installers only update the software, but neglect precompiled headers.

But fear not, the fix is easy. Just head to your friendly neighborhood Terminal.app and enter "sudo fixPrecomps -force" and MacOS X will try to update all precompiled headers.