I must say I am not keen on changing the light when you enter the scene. Enter a cave and everything becomes dark, outside included if you turn around, which is obviously wrong. Also the cave would still look wrong from the outside looking in. It would only work in a enclosed space with a blocked entrance/exit.
The only option that works at all is baking, but I have my issues with that as it generally does the exact reverse of it's purpose (speed things up) The memory use is just too high as well. I've tried black lights which do nothing, which makes sense in reality of course, but not useful here.
I wonder if we will ever see proper lighting effects where shadows do not become paler on floor objects rather than terrain, and shadows cast in shadow are actually darker etc. I must admit if performance work goes well, this would be a good area to improve next.
SPECS: Q6600 CPU. Nvidia 660GTX. 8 Gig Memory. Win 7.