In one example (I hunted out in the video) at 1/1000th second the front curtain is moving faster than the rear curtain and looks like it would overexpose the bottom of the frame slightly.

*timeshift

Adding video to channels (Timeshift) - requested by ant.

And that's why you have a slowest "flash sync speed" - if you shoot at faster shutter speeds than the flash sync speed (typically 1/200s-1/250s), the whole sensor wouldn't be exposed at once, and the lower part of the frame would be dark.
*related=http://videosift.com/video/High-Speed-video-of-Canon-DSLR-Shutter-Smarter-Every-Day
*related=http://videosift.com/video/Ultra-High-Speed-Video-of-Nikon-D3-Shutter-Action

High Speed video of Canon DSLR Shutter - Smarter Every Day has been added as a related post - related requested by Zawash.

Ultra High Speed Video of Nikon D3 Shutter Action has been added as a related post - related requested by Zawash.

Canon manages it by strobing the flash repeatedly though the moving slit. This is called HSS flash as I recall.

It lets you shoot super fast exposures, get huge apertures and still use some fill.