re: For astrophotographers
By doing away with the grey LCD display on the camera for exposure/apeture etc. and using the very bright LCD screen on the back with black text on a bright white screen (even if you turn it down), both the 400D ( which I have and love ) and the 450D are both excellent cameras, and excellent means of screwing with dark adapted vision. If I was looking for a cheap Canon DSLR for Astrophotography, I'd pick up an old 350D.
Doug