Canon came to the Auto ISO game later than most, and the early implementations were not as good as other manufacturers. On the Rebel T2i, it's a lot better, but one annoyance.
The Auto ISO is set to on most of the time on the T2i, and it works very well most of the time, except when you have a long lenses mounted. The reason? The minimum shutter speed is set too low. Often, when a 50mm lens is mounted, the shutter speed is set at 1/60 of a second, and this in some cases are too low when you multiply the focal length by 1.6. It should be around 1/80 to 1/125 of a second to avoid camera shake. Sure, you can use manual mode, but it's not as fast as AV mode, which I use almost exclusively.
Obviously, this can be enhanced by making Auto ISO shift-able. We already have shift-able Program mode, why not implement this in Auto ISO? Allow the shutter speed to be changed while maintaining the same aperture, and thus making the ISO change its value to compensate. This can even be done with a firmware update, but I doubt Canon will do it. Hopefully the next Auto ISO version will have this feature.
While we are on the topic of Auto ISO, as I have already mentioned before, the ISO can be incremented in 1/3 stops. But, this can not be done manually. The ISO can only be changed in full stops. This is puzzling move on Canon's part. Clearly, it's not a pro feature to be able to set ISO in 1/3 stops, as you can do that with the 30D, 40D, 50D, etc.