MarkB
Legend
I guess the real question is:
Does darkness actually add anything to your game?
Without heavy modifications to the existing rules, I doubt it. The light rules as written are basically presented as a big screw to use against the PCs if they neglect to write "a light source" on their character sheet.
If you're going to go all out and actually work out where shadowy areas on a battlefield are, and actually play monsters as if they don't want to stumble about in the dark all the time, then I'd say feel free to enforce accurate usage of light sources along with it. I think such a system has the potential to add a lot to the game (ok, if you move over there with our light source, then I have concealment because I'm in the shadows from that statue, so I can hide!) but also a lot of complexity.
This is one area where I was really looking forward to the DDI Virtual Tabletop. One thing that such a system could do really well would be to simulate the effects of light sources and shadows automatically, in real-time, based upon what light sources each creature on the board is carrying. You'd get your added tactical dimension, without the complexity.