Eh, it's not saying the whole design will be flat (like the current iPhones), just that the screen will be flat. Sides could be just as rounded as they are now. If they want to make it impact resistant, they could go a long ways in that direction simply by having a (strong sapphire covered) screen inset a couple millimeters into the watch, so it has a 1-2mm lip around the outside edge of the screen, so no side impact or edge impact would hit the screen itself - you'd have to have something impact the center of the screen (have it hit by a corner, or a rounded object of some sort).