Alright, so here's an opportunity to max out the 5DII and push the gear to its limits: Hiking down the side of a mountain at night with the path lit by our headlamps (very helpful for focusing, btw). It is a cloudy evening so there's no moonlight to come through. The only glow in the sky is from the very little light pollution bouncing off the clouds.
Crank up the settings on the camera to ISO 25,600 and f2.8 and let's see what we get! This was taken at 1/8". Drop the shutter speed much lower and either the hikers get blurred walking by or the landscape gets blurred as I pan the shot. We're basically hitting the upper limits of this setup and I'm noticing I'd like to go further! A 1DIV with its ISO 102,400 sounds really appealing right about now... I was wondering when you'd need such an extreme ISO and now I know! :D
This image was hit with some noise reduction in ACR as well as a second helping by noise ninja in post.