Via Canon Rumors.
Canon has released two new firmware versions for their EOS 1D Mk IV and 1Ds Mk III models, adding compatibility with their new EF 200-400 f/4L IS 1.4x lens. The lens has a built-in 1.4x extender, which makes it a 280–560mm f/5.6 lens. In order to correctly operate when the new lens is attached, the two cameras need the latest firmware installed. In addition, Canon adds a bugfix to the 1D Mk IV firmware correcting an issue with RAW files in continuous shooting mode.