Cannon fixes the problem on 7D with a new firmware release:
Firmware Version 1.1.0 incorporates the following fixes.
1. Corrects a phenomenon that in images captured by continuous shooting, and under certain conditions, barely noticeable traces of the immediately preceding frame may be visible.
Firmware Version 1.1.0 is for cameras with firmware up to version 1.0.9. If the camera's firmware is already version 1.1.0, it is not necessary to update the firmware.
The new firmware can be downloaded from here.
Via canon rumors.