Here's MY experience with ISO on the T2i:
The camera arrived in the mail yesterday, was charged, and I took three pictures by firelight, something my XS couldn't handle.
Then today the real test came. I teach at an elementary school, and our cafeteria/auditorium is horribly lit. Today the students did poetry recitation up on the stage with kids and parents in attendance.
Normally it's a bear to get anything decent using my XS and the 55-250 in the school auditorium, and I am the school photographer. I take pictures of all the programs and such. That lens, of course, isn't known for being a low-light lens, but that's what I have for now.
This new T2i is blowing my mind. I shot at 3200 ISO and got great pictures. I'll post later when I get home (my work computer blocks Smugmug). I am very impressed.
If someone wants to say that the T2i doesn't have better ISO performance than the XS they are welcome to continue thinking that. They can even be right and I might be in la-la land.
I like la-la land.
Thank you. 