Firmware version 1.0.8 incorporates the following fix.

Fixes a phenomenon in which captured images may become overexposed when using the camera's built-in flash or an external Speedlite in combination with the lenses listed below:*
a) EF300/4 L IS USM
b) EF28-135/3.5-5.6 IS USM
c) EF75-300/4-5.6 IS USM
d) EF100-400/4.5-5.6 L IS USM

Firmware version 1.0.8 is for cameras with Firmware Version 1.0.5. If the camera's firmware is already Version 1.0.8, it is not necessary to update the firmware.