when I used to shoot with my HV20, I had four batteries, the basic size. This would last me all day. I haven't tried the T2i in the cold yet, however I would imagine it would be about the same. Now this is basically about ten hours of shooting.
I haven't had any problems with the no-name brand batteries off ebay. Saves a ton of money.
On a second thought, I always thought about crocheting a little baggie for the camera to sit in like a little jacket for it to stay warmer haha.
I only have one, and it's somewhat of a neusance having to warm it, but if i keep warming it it'll last a whole day. for cold, I would like to have 2. One to have being warmed and one in the camera. That would easily sustain a T2i for a whole day, if i switched them every time the battery froze. In warm weather one battery lasts me two days of filming.
yeah the camera will fog up pretty bad if you move it from cold to hot fast. just put the camera in one of those big zip lock bags before you go inside, and wait until the its about room temperature before taking it out. that usually keeps most of the moisture out.
DSLRs actually have problems overheating while shooting for long periods of time. Cold will affect the battery, not the camera. The foot warmer probably wouldn't keep the battery much warmer, and it might add to any overheating your camera has.