From the firmware release notes:
Firmware Version 1.2.0 addresses the following phenomenon:
- If pressure is applied to the lens barrel while the lens is mounted to the camera (pressure can be applied even while attaching the lens cap or while carrying the camera with the lens attached in a bag), the autofocusing function of the lens may stop working.
Lenses with the following serial numbers are equipped with Firmware Version 1.1.0:
- The third digit in the serial number is either 0, 1, or 2 (xx0xxxxxxx, xx1xxxxxxx, or xx2xxxxxxx).
You can download the firmware from the Canon US Website.