Had noticed the contrast difference with HDR on (Single GPU and SLI) in Doc Mitchell's house. Got to REPCON and it's way worse I'm going blind with the intense changes in light levels.

http://i1189.photobucket.com/albums/z428/BladeFireLight/ScreenShot1.jpg Well lit

http://i1189.photobucket.com/albums/z428/BladeFireLight/ScreenShot3.jpg Poof! the lights are out.

Verified this with vanilla, so this was not related to a mod.

It might be similar to a lighting bug I have occur sometimes after I visit the Lucky 38 - which persists wherever I go after that.

The ONLY way I have been able to fix it is to:

Save (in a new slot just to make this easier)
Exit the game
Load a save game before I encountered the glitch OR create a new game
Reload the save where I had encountered the glitch - and it's not fixed

If that doesn't fix it then it is probably a completely different bug.