What’s true for doctors applies equally to consumer electronics manufacturers: first, do no harm. Canon is usually pretty good at adhering to that philosophy, making only minor changes to successful products and saving the daring moves for the models that need it.
Now, changing sensors isn’t normally considered terribly daring when it comes to digital cameras. But when its predecessor — in this case, the Canon Digital Rebel XT / EOS 350D — was renowned for producing excellent, low-noise photos at a more-than-adequate 8-megapixel resolution, it’s risky to replace it with a higher-resolution but potentially lower-sensitivity chip as Canon did with the Canon Digital Rebel XTi / EOS 400D.
Perhaps the Nikon D80 upped the stakes; perhaps Canon felt it was an inevitable necessity. Whatever the reason, it yields mixed results.