Clang 3.1: interesting warning...

gistfile1.m

@protocol MyProtocol <NSObject>

@end

int main(int argc, char *argv[])
{
    @autoreleasepool
    {
        id <MyProtocol> foo = nil;
        __unused id value = [foo valueForKey:@"bar"];
        
        return EXIT_SUCCESS;
    }
}

I understood :) http://unixjunkie.blogspot.com/2008/03/id-vs-nsobject-vs-id.html