I am developing on mobile devices, and the ability to determine if the game still has focus is extremely important. The game can be interrupted by users pressing the back or home button, or when receiving an incoming phone call or when another app comes into the foreground. Having this ability will allow developers to pause the game, save the current state or do other stuff that is required when the game is forced into the background.
I believe that this should be a core feature, especially when targeting mobile devices.
Please make this happen. (I'm down on my knees begging)
Use the Browser object's On suspended/On resumed triggers. The runtime automatically pauses when going in to the background.