My 400D has arrived and I've started trying it out for night photography. My initial reaction is that as long as you use the "noise reduction" mode the images are very good indeed. An example:
http://www.nightfolio.co.uk/subpages/as006.html
(There is a link at the bottom of that page to a larger version of the photo). Incidentally I am also trying out the Tokina 12-24mm f4 ATX zoom which strikes me as being a very special lens for not alot of money (relatively!), at f5.6 the corner sharpness is incredibly good imho, the build quality is very high indeed.