Dunno, I think it will come much faster. Local dimming is not expensive/complex to implement, basically more advanced signal processing.
Because LEDs have a fast response times, their brightness can be changed very quickly (unlike CCFL), and I think almost all LEDs do average dimming already (alter illumination bightness on all LEDs together based on avg brightness of the whole frame). This is how they achieve deeper blacks than LCD.
Local dimming just takes this a step further, manipulating individual LED's brightness based on darkness of the corresponding area of the frame. From my understanding edge-lit LEDs are only illuminated from the left/right edges so local dimming is only possible in the horizontal plane e.g. when displaying a letterbox picture the LEDs driving the lines where the top/bottom black bars are can be switched off completely.