Owning the XSi myself, and having a couple friends who own the T1i and T2i, I can state that there is definitely an improvement in image quality going from the XSi to the T2i. The technology is several years improved on the T2i, as the XSi uses much older sensor tech and more basic image processing chips. The XSi does not perform well at all when moving beyond ISO 400, and ISO 1600 is practically unusable for low-light photography. It only really performs decently when you have enough light to use a lower ISO anyway. In contrast, the T2i performs quite well at ISO 1600, and does decently well at ISO 3200.
My friends and I do quite a bit of night sky photography, and there is no question that for short-exposure night sky photography of the milky way and constellations, the T2i does FAR better than the XSi. My camera (the XSi) is unusable at ISO 1600, but ok at ISO 800. Even with a wide 50mm f/1.4 lens, its pretty tough to get a decent milky way shot with the XSi. With the T2i, getting good milky way shots is much easier as ISO 1600 is perfectly usable, and longer shots, up to 25-30 seconds, are a breeze. For star trails, there isn't much of a difference, as you use a low ISO and stop down the aperture a bit anyway, but the T2i offers quite a bit more resolution, so its still a win.
I do a lot of moon photography (partly because I love the moon, and partly because the moon is the only thing visible when I have time to do any photography these days -_- ). The XSi fares decently, however its limited dynamic range does pose a problem at times. The T2i is a tad bit better in the area of dynamic range, although not a whole lot.
Regardless of the kind of long exposure photography you hope to do, night sky photography is an excellent test. Comparing the XSi with the T2i for night sky photography, the T2i is definitely an improvement.