It's always tempting to buy a DSLR for astronomy and some low light snapshots. Starting from the launch of 300D, I was thinking. The file is nearly closed with the discovery that 400D is a bit too noisy for astronomy purpose. With the re-launch of 350D, the file is open again but then it's only available as kit, that makes the decision more complex. And now with the announcement of 40D, the problem is even more...
First of all, 40D is not going to be very good in terms of noise, I expect it's slightly better than 400D at most, but it comes with live preview. Live preview should be pretty nice for astronomy purpose, and the new one from Nikon also supports live preview and I believe that it will become a standard feature after one or two years.
In terms of noise, it's closely related to the pixel density, but it seems like it's growing without stopping, that's the real trouble. I mean if you want to wait one or two years for live preview, you're going to get a sensor with heavier noise due to the pixel war.
The current best choice should be:
kit lens for daily snapshots (forced)
EF-S 10-22mm for milkyway and natural scene
EF 200mm f/2.8 for star field
really need to plan for the solar eclipse next year, should make the purchase within 2007 to make myself experienced before going.