The latest Nvidia 378.66 and Blender 2.78b
Blender User Preferences → System → Cycles Compute Device
shows “None”. CUDA devices for Cycles rendering can not be selected.
Windows 10
64 bit
4 x Geforce GTX 1080
Nvidia driver 378.66
Blender 2.78b
When I roll back to Nvidia driver 376.33 it works. I can select all of my four Geforce 1080 GPUs for rendering.