According to IMDb:
" At the height of WW2, a group of high-ranking German officers hatched a plot to assassinate Adolf Hitler, and seize power of the military command in order to end the war. The operation was codenamed "Valkyrie", for the emergency plan that was meant to be used in case of a revolt against the Nazi government. This plan had been modified by the conspirators to ensure their success, but for various reasons the plot failed when finally carried out on 20 July 1944. The conspirators of the inner circle were shot after a kangaroo trial or sentenced to death soon after. "
Many of us may know of this as the July 20th, 1944 Wolfschanze (Wolf's Lair) Headquarters explosion. Colonel Claus von Stauffenberg was the officer who took the bomb into the bunker.
http://valkyrie.unitedartists.com/ (says its due out Jan 30, 2009)
As for the trailer, I went smart, and went to the gallery. There is a Kubelwagen and a Pz. 4, but the tracks look odd, and from the side it looks like they rigged a panel to hide the real tank suspension. Also saw either an Sd.Kfz. 250 or 251 halftrack (could only see the front) and I think I saw the open protective cage (open) of a 222 patrol car!
Look through the site's Navigation link, as the 'Historical' section (still being filled in) is rather informative about the whole situation, and locations.