I've had this in my vimrc for a little while:

  let hr = (strftime('%H'))
  if hr >= 17
      set background=dark
  elseif hr >= 7
      set background=light
  elseif hr >= 0
      set background=dark
  endif

Using a light sensor would be cool, but I actually work from a desktop PC most of the time, so no sensor. I guess the next best thing would be something like f.lux does: query some web service for the sunset time at your current location.