You have stated the conditions that you would be shooting in and it all boils down to the question on how well is the sealing of both lens and camera.
Some would say that the best sealed brand for DSLRs are Olympus.
But if you are comparing the two, Nikon and Canon.... I would have to say for now Canon would ensure the sealing that you require. However, only the 1 series bodies (1D Mark II and 1Ds Mark II) would be in this range. 20D might not be up to task because the body might not be as heavily sealed as the 1D Mark II. I would say that as long as you don't change lenses while you are out in the desert, your CMOS sensor should be ok.
For all the Nikon people, I really don't know how well the pro series lenses are sealed. If any of you know, please do let me know about that.