Is [UIScreen mainScreen].bounds.size becoming orientation-dependent in iOS8?

Yes, it's orientation-dependent in iOS8, not a bug. You could review session 214 from WWDC 2014 for more info: "View Controller Advancements in iOS 8"

Quote from the presentation:

UIScreen is now interface oriented:

• [UIScreen bounds] now interface-oriented
• [UIScreen applicationFrame] now interface-oriented
• Status bar frame notifications are interface-oriented
• Keyboard frame notifications are interface-oriented

Yes, it's now dependent on orientation.

I prefer the below method of getting the screen size in an orientation-independent way to some of the answers above, both because it's simpler and because it doesn't depend on any of the orientation code (the state of which can be dependent on the time that they are called) or on version checking. You may want the new iOS 8 behavior, but this will work if you need it to be stable on all versions of iOS.

+(CGSize)screenSizeOrientationIndependent {
     CGSize screenSize = [UIScreen mainScreen].bounds.size;
     return CGSizeMake(MIN(screenSize.width, screenSize.height), MAX(screenSize.width, screenSize.height));
}

Yes, indeed, screen size is now orientation dependent in iOS 8. Sometimes, however, it's desired to get a size fixed to portrait orientation. Here is how I do it.

+ (CGRect)screenBoundsFixedToPortraitOrientation {
    UIScreen *screen = [UIScreen mainScreen];

    if ([screen respondsToSelector:@selector(fixedCoordinateSpace)]) {
                    return [screen.coordinateSpace convertRect:screen.bounds toCoordinateSpace:screen.fixedCoordinateSpace];
    } 
    return screen.bounds;
}

Yes, it's orientation-dependent in iOS8.

I wrote a Util method to resolve this issue for apps that need to support older versions of the OS.

+ (CGSize)screenSize {
    CGSize screenSize = [UIScreen mainScreen].bounds.size;
    if ((NSFoundationVersionNumber <= NSFoundationVersionNumber_iOS_7_1) && UIInterfaceOrientationIsLandscape([UIApplication sharedApplication].statusBarOrientation)) {
        return CGSizeMake(screenSize.height, screenSize.width);
    }
    return screenSize;
}