Well, there are definitely lighter areas. I have kept those a secret because I think they might count as spoilers. Just watch though, the first review will talk about it as part of the plot and I'll be kicking myself in the butt for not releasing images of those areas because I didn't want to give anything away.
I'll see it I can put in some gamma control as well. This is one of those things that are screen specifc. On my sisters PC the same scenes looked like they were lit up with sunlight. You are underground and there are lots of dark shadows, but at the same time I tried to make the important bits of the scene easy to spot and stand out.
An experiment you can do is to save one of the screens to your harddrive, then pull it up in Windows picture viewer. Hit the slideshow button and it should then display just the image and turn the rest of your screen black - that's pretty much how it will look in the game. These light colored backgrounds on the web pages do make them seem even darker. They ARE dark, but not so dark that you can't see anything.
Which screen in particular is the darkest in your opinion?