code:

// in MyAppDelegate
- (void)showLoginWindow:(LoginViewController *)loggy
{

    NSLog(@"my root viewcontroller is: %@ and loggy is %@", self.window.rootViewController, loggy);
    // above shows what I expect

    // [self.window.rootViewController presentModalViewController:loggy animated:YES];
    //above works in iPhone and iPad landscape.

    if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad) { 
        UISplitViewController *splitty = (UISplitViewController *)self.splitView;
        NSLog(@"Splitty is %@", splitty); 
        [splitty presentModalViewController:loggy animated:YES];
        // above works fine in landscape
    } else {
        [self.window.rootViewController presentModalViewController:loggy animated:YES];
    }
}