From Canon UK Consumer Product Support today:
We have learned that some EOS-1D Mark III and EOS-1Ds Mark III digital SLR cameras have a problem in the accuracy of the AF (autofocusing) feature.
If an AF point other than the center AF point is used, focus may become soft.
Free AF accuracy checks and adjustments will be provided to affected products. Please click here to complete a registration form if you own an affected product and wish to arrange for service. After you have submitted the form you will be contacted by a representative of a Canon Service Centre within one week for a service appointment.
A prior registration and appointment is preferred before sending your camera to Canon. Any cameras received without a pre-registration and appointment will be serviced at the earliest possible opportunity.
Based on the comments and suggestions of professional photographers, we have also created supplementary AI Servo AF information (AI Servo Auto Focus Manual) for users to fully utilize the AF capacity of the cameras. Please refer to this information when you shoot with the EOS-1D Mark III and EOS-1Ds Mark III cameras.
Previous information regarding AF (Auto-focusing) with EOS-1D Mark III and EOS-1Ds Mark III:
The following web announcements were previously released to inform users about improvements in the AF accuracy of the EOS-1D Mark III and EOS-1Ds Mark III.
EOS-1D Mark III: Start of Repairs for AF Mirror Adjustment Problem
Release of firmware that improves the stability of AF accuracy in AI servo AF when shooting extremely low-contrast subjects
EOS-1D Mark III - firmware version 1.2.3.
EOS-1Ds Mark III - firmware version 1.1.2.
The above information, together with the provision of free AF accuracy checks and adjustments, allow users to utilize the AF function of the EOS-1D Mark III and EOS-1Ds Mark III with greater assurance.
We offer our sincerest apologies to customers using these products who have been inconvenienced by this issue. Canon always strives to provide the highest quality products to our customers, and we spare no effort in our quality management to make sure our customers can use our products with confidence.
We hope our efforts will earn your understanding.