you might try setting the window alpha to zero, but the app won't actually sleep until the phone goes to sleep by itself. Then you have to figure out how to know when to reset the alpha which is probably a bigger problem! Lest your user tries to wake their phone and is presented with a blank screen...
Setting the window alpha won't do what you are thinking it will do. YOu can not turn the screen on/off because it is against the SDK rules. Another option would be to turn the screen brightness down (GraphicsServices), but that is also against Apple's rules.
Basically, there's nothing you can do without barring an AppStore release.
it might be easier to set the alpha of each of your views to 0, rather than the window itself. I think (haven't tried this) that you can blank out all of the views by setting the alpha of the root view. Make the window.backgroundColor black... :-)
Turning iPhone screen off or sleep
How to acces to window's alpha parameter from the view?
Linear Mode
