Is the 28mm lens sufficiently wide enough on the 40D to capture reasonable aurora borealis photographs? Given the 1.6 crop factor it seems like it might not be (which would probably mean that I look at either an off-brand lens or rental), although so far my research into this question hasn't really led me to an answer yet.
From what I have read, to shoot good photographs of the aurora borealis that capture as much of it in the frame as possible (such as in this shot), you need a wide, fast lens (and luck), but most of what I have been reading is from a full frame perspective.