gistfile1.txt

    Class newClass = NSClassFromString(@"UINewClassThatYouCantHave");
    if (newClass) {
        NSLog(@"We're on an iOS that has the new class");
        id newObject = [[newClass alloc] init];
        if ([self respondsToSelector:@selector(selectorThatUsesNewObject:)])
            [self performSelector:@selector(selectorThatUsesNewObject:) withObject:newObject];
    } else {
        NSLog(@"Class doesn't exist!");
    }