My solution to this problem is to replace self.view before setting canDisplayBannerAds=YES with a UIView that overrides layoutSubviews.
@protocol LayoutViewDelegate <NSObject>
- (void) layout;
@end
@interface LayoutView : UIView
@property (nonatomic, weak) id<LayoutViewDelegate> delegate;
@end
@implementation LayoutView
- (void) layoutSubviews {
    [super layoutSubviews];
    if (self.delegate) [self.delegate layout];
}
@end
I do this replacement in viewDidLoad:
- (void)viewDidLoad
{
    [super viewDidLoad];
    NSLog(@"Calendar.viewDidLoad");
    // Replace the view, before setting up ads for iOS7, so we can get callbacks for viewDidLayoutSubviews; otherwise, we only get viewDidLayoutSubviews callbacks when ad disappears.
    if ([Utilities ios7OrLater]) {
        self.layoutView = [[LayoutView alloc] initWithFrame:self.view.frame];
        self.view = self.layoutView;
        self.layoutView.delegate = self;
    }
}
And in viewDidAppear, I do:
- (void) viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    if ([Utilities ios7OrLater]) {
        self.canDisplayBannerAds = YES;
    }
}
And I add the delegate method:
// This *always* gets called when the banner ad appears or disappears.
#pragma - LayoutViewDelegate method
- (void) layout {
   // do useful stuff here
}
#pragma -