Canon has annunced a firmware update for the mid-range Reflex Eos 550D (Rebel T2i, Kiss X4). It fixes a phenomenon in which tone jumps become noticeable in some images, depending on the shooting scene, when shooting with the Auto Lighting Optimizer settings (Low / Standard / Strong). For example, tone jumps will be corrected in scenes such as a dark sky just after sunset, in low-contrast areas, depending on how bright those areas are.

You can download the firmware from this official page.