High ISO noise reduction

The EOS 40D, 1D Mark III and 1Ds Mark III all feature a Custom Function for high ISO noise reduction. This can be used on images shot at high ISO speeds to help reduce the appearance of noise in the final image. If the Custom Function is turned on, some noise reduction will be applied at all speeds, but it will be most effective at higher ISO settings. If you do set high ISO noise speed reduction, you will notice that the burst rate is dramatically reduced due to the extra processing required in the camera. In the EOS 40D, the maximum burst will only be 8 shots regardless of the file size recorded.
EOS-1 series
Camera settings
Custom functions