[SDK2003]

Have you guys separated focus and metering from the shutter button ?

On my 20D I use the * button on the back of the camera to focus - I keep it held for focus tracking.
The shutter button sets the exposure at the time the shot is taken and it doesn't lock the exposure. So as I'm panning the camera with the shutter button held down the exposure is updated with the changing light conditions.

If you have focus and exposure set on the shutter button then the moment you hold the shutter button to start focus tracking the exposure is also locked and I guarantee that the light conditions will be different when you actually take the shot.