This is one feature that I was hoping for!
From DPReview
Auto ISO Improvements
The 5D Mark III's Auto ISO mode now offers additional customization options. You can now set a minimum ISO as well as a maximum, and define a specific minimum shutter speed too. It's not quite the same flexibility as offered by the Nikon D800 with its multiple progam lines, but it's a big improvement over previous Canon implementations. And, unlike the 5D II, can be used in Manual shooting mode, allowing you to specify the shutter speed and aperture you want, and leaving the camera to worry about ISO. You can apply AE lock when working with Auto ISO in Manual mode and the viewfinder will show you the difference between the locked value and the current metered value. It's not currently clear whether there's any way of setting exposure compensation.