the demo has been updated -
"The updated demo for Darkest of Days fixes a major bug where the game would crash while loading levels. In Darkest of Days, players visit some of the world's biggest conflicts to change the course of history. The demo includes Custer's infamous last stand at the Battle of Little Big Horn and also to the Battle of Antietam during the American Civil War. System requirements are shown after the jump. The full game is expected to release in September."
Wonder if it now works with the new 190.56 drivers too...