Canon has release a new firmware for users of 550D (aka T2i). The new version addresses the issue on manual exposure shifting when shooting videos.
Fixes a phenomenon in which the set aperture moves unexpectedly when shooting movies in manual exposure mode using some Canon lenses (such as macro lenses).
The Version 1.0.8 firmware is for cameras with firmware up to Version 1.0.7. If the camera's firmware is already Version 1.0.8, it is not necessary to update the firmware.