The Digital Rebel T2i has a "normal" ISO range from 100 to 6400, whereas the T1i had a range of 100-3200. The T2i ISO range can be expanded to include ISO 12800 (H) via a custom function. In the T1i the expended range was 6400 (H1) and 12800 (H2).
The reason why Canon (and other camera makers) offer a "standard" and "expanded" ISO range is twofold. First the "normal" range should give good results and low noise, whereas the "expanded settings" may be lower quality. However there is a more technical explanation. In the "normal" or "native" settings, ISO is determined by the gain of the analog amplifiers between the sensor output and the analog-to-digital converters. In the expanded range settings, the effective ISO is obtained by a digital technique which could be considered equivalent to "pushing" film. For example if ISO 1600 is the highest normal settings, then an ISO 3200 "expanded" setting might be obtained by under-exposing the sensor by one stop at ISO 1600 and then digitally processing the resulting image to increase the brightness up to a level that you'd expect from ISO 3200. Just like with film this results in somewhat lower image quality, noisy shadow detail and a small loss of dynamic range.
The noise levels I measured in Digital Rebel T2i images was very similar to the levels I measured for the EOS 7D. This isn't too surprising since both cameras share a similar 18MP sensor, a Digic IV processor and presumably similar image processing and noise reduction software.
The effect of ISO on resolution is shown in the following image set:
Overall noise levels are low. ISO 3200 is certainly quite usable and even ISO 6400 is pretty good if you add in a little extra noise reduction and print small. ISO 12800 1s best saved for emergency use only as noise increases significantly and resolution is reduced.
Dynamic range is related to image noise because high levels of image noise mean that you lose information in the deep shadows. Dynamic range is basically the range in stops between something that doesn't record as absolutely black (0) to the level that's just below absolutely white (4096 for 14-bit depth). The noisier the shadows are, the more difficult they are to tell from the level you get with no light at all, and so the smaller the dynamic range of the shadows. Using noise reduction has the effect of increasing dynamic range somewhat.
Though I didn't do a scientific evaluation of dynamic range, the Dxomark.com website did. DxO is probably the leading company in the digital camera measurement software business and a maker of one of the leading RAW image converters. Their measurements put the DR of the Digital Rebel T2i, 7D and EOS T1i within a few 1/10s of a stop of each other, at around 11 stops at the ISO 100 setting and 6.25 stops at the ISO 12800 setting.