Canon released a firmware update for the Canon T2i and 550D which 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).” It is always a good idea to keep your camera up to date so I recommend to all owners to update.

You can find the firmware here: http://web.canon.jp/imaging/eosd/firm-e/eosdigital7/firmware.html