I took this screenshot from the explorer bridge - it was something I never noticed until now.
http://img4.imageshack.us/img4/8290/screenshot2010021118503.jpg
There is what appears to be a shadow texture on top of the regular one and it only shows up on some interior areas rather than everywhere. Changing the graphical settings doesn't appear to affect it in any way, shape or form, though as I've not noticed it until now, I'm wondering if it's something new or it was always there. This on a GTX 295 with 190.62 drivers. Also tried it with 196.21 but that broke the cursor.
Edit: Just figured out what's causing it, it's the ambient occulsion option. Since it seems to be making everything look dirty rather than more shadowed, I think I'll keep it switched off in future.