Will a canon lens fit on my Nikon d40?

All lenses with Nikon mount are compatible with Nikon D40. If the question rephrase : "What lenses are able to autofocus with Nikon D40", then the answer is : the new Nikon AF-S lenses (with Silent Wave Motor inside), Sigma lens bearing HSM code, and upcoming Tamron lenses (18-200mm, 17-50mm and 70-300mm).