Here is another "useless" (LOL) test comparing the very latest Nikon & Canon bodies with the 40D thrown in for good measure. It's worth reading if for nothing else other then to see the different ways Canon and Nikon apply noise reduction in camera and how it affects detail.
http://www.prophotohome.com/Nikon-D3x-Canon-5D-MKII-High-ISO-Image-Quality-Review