40D also has a bigger view finder, a max shutter speed of 1/8000 vs. 1/4000 on the xsi, an x sync of 1/250 vs. 1/200, a larger buffer that will shoot up to 75 jpg and 17 raw as 6.5 fps before slowing to 1fps as opposed to the XSI's 53jpg and 6 RAW at 3.5fps before slwoing to 1 fps, Kelvin white balance (meaning you can set the actual color temperture in degrees Kelvin), the control wheel on the back with the ability to change aperture with a dedicated control rather than having to press a button and then scroll the shutter speed wheel, top mount LCD, it takes CF cards vs. SD and higher capacity battery rated at 1100 shots vs 600.
All that was on the charts at the bottom of the Bob Atkins article that was posted. Oh...and it's a metal body vs. plastic.
Ed: And if the XSI is the same as the previous rebels, you have to actually use the camera's menu to change the ISO instead of being able to change it with a button press and the control dial. The 40D should has have a quicker AF. There's probably some more. Do some reading.