Forgive me for raving about a new camera, but I am so pleased!
I recently bought a 40D principally for its high ISO speed. My existing 400D was disappointingly noisy at ISO 1600 even with in-camera noise reduction.
The 40D has performs well in my opinion even at ISO 3200 (with noise reduction ON). Having never used a 30D I can't comment if the new camera improves on the old in this respect.
However, there is one new feature in the 40D which I feel is a killer advantage for night photography, it is the new Live View focussing mode. I saw an excellent video on the net which explained this:
http://www.cameralabs.com/reviews/Canon_EOS_40D/live_view_astro-photography.shtml
Using the Live View mode focussing feels like using a view camera ground glass using a high powered loupe, very precise. In fact it is just like the precise focussing possible with an astronomical telescope. Using fast wides at full aperture I have done some basic tests focussing on distant streetlights and also the bright summer star Deneb and the results are incredible. For the first time I feel I can accurately focus at night.
Having recently abandoned Nikon film for Canon digital, focussing has been a problem for me. Autofocus was useless, and the 400D viewfinder is dim and without a proper split screen. My Canon 24mm f1.4 gave very hit and miss results at full aperture and focussing by scale, I was disappointed. Using Live View even at full aperture this lens now produces crisp and sharp star fields, I am very happy indeed with my purchase.