Look in your menu for something called 'Safety Shift' (I don't know where it would be on the 5D).
Basically, 1/800s didn't give you enough light with iso200, even with wide-open aperture. If the camera hadn't adjusted it to 1/20s, you would have a black photo.
I always use Tv (around 1/400s) on my 7D in conjunction with AutoISO. The aperture pretty much always stays wide-open (because i'm using a slow lens, 70-300L), the iso goes between 100 and 3200. As it gets dimmer in the evening, the iso hits 3200 then the shutter speed starts going down. Only in the brightest sunlight (ie never), it stays on iso100 and the aperture might go up to f/8.
I'm not sure how well the 5D2 handles AutoISO though, so you may be out of luck and have to go back to Av wide-open and set ISO manually based on the light...