From what I hear, this is a great camera.
The Nikons (D3, D300 & D700) are kicking Canon atm.
Canon's released the 1000D as an entry body for those that want to move up from P+S cameras, but unless Canon can come up with something better than the 1000D/450D/40D, they'll loose ground to the new Nikon range.
In my opinion, I think these new Nikons have superb low light performance for terrestrial so they should be great for astro too.
The only Canon pro-sumer model that has exceptionally better low noise performance over the other Canons atm is the 5D (unless you move up to the 1D bodies) because of it's FF sensor and they haven't tried to cram in too many megapixels on the sensor.
Once they start packing the sensor with higher pixel count they introduce more noise.
It will be interesting to see what Canon will do now, because there's speculation of a new 5D MkII (or equivalent) to be released soon.
For now, if you haven't already got Canon lenses, I'd highly recommend the Nikon bodies.
If you want to wait or already have Canon gear, then hang on and see if there's any announcements from Canon in the next month or two.
This will also give you a better idea of how the D700 is performing not to mention that prices usually go down, not up on camera bodies.
The Canon 5D is around $2350 which is a bargain for this superb FF camera and some people here are using them for astro with fantastic results.
The Nikon D700 is around $3600, but again should be a great performer.
Good luck and keep us posted on your decision.