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.
The firmwares can be downloaded here (1D Mk IV) and here (1Ds Mk III).