Canon released a firmware update for the EOS 60D today, fixing a couple of issues.
Firmware Version 1.0.8 incorporates the following fix.
1. 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