This is a convention introduced by Apple: http://developer.apple.com/library/ios/#documentation/2DDrawing/Conceptual/DrawingPrintingiOS/SupportingHiResScreensInViews/SupportingHiResScreensInViews.html#//apple_ref/doc/uid/TP40010156-CH15-SW8
This convention is also used by this client side Javascript library: http://retinajs.com/ so from my point of view is is not a good idea to make this configurable.