Nikon is developing an X100-esque hybrid viewfinder for DSLR cameras. A recently published patent filing by the company describes and shows a DSLR with a viewfinder that can switch between optical and electronic modes. The purpose appears to be for providing the photographer with an optimal view of the scene — if the scene is too bright for the photographer (e.g. the camera is pointed towards the sun), then the camera switches to an electronic view that provides the user with a view that’s more suitable for their eyes.
Unlike the X100’s viewfinder, however, there is no electronic image overlaid on the optical one when in OVF mode.