The reason the D200 dark frame looks so good is:
1. Its well known that Nikon apply a median-type filter to long exposure images.
2. Nikon do not apply a brightness offset to raw images (unlike Canon), this means the dark frame will look much darker with the Nikon.
Both these actions damage fainter detail in an astronomical image.
As a comparision I have attached a dark frames from my 350D, 1020secs at ISO1600 (at 24c) process 2 ways. Version 1 is a straight raw, but with the image offset is brought back to 0 to match the Nikon.
The second image has both 1) 0 offset and 2) a median filter comparable to the built in filter of the Nikon. I have then increased the contrast 10x (i.e push processed the image 2.5 stops in film speak) otherwise it would appear black.
Sorry I disagree, it is not hype that Canon have made a great achievement with their sensors. The noise levels and quantum efficiency are as good as any CCD sensor (its not fair to compare specialised monochrome sensors which have very high Quantum efficiencies but are too expensive/impractical to be used in a digicam).
As a last comment compare this 15 minute exposure of M83 made by Bill Christie using a 350D and an 8" f4 telescope to your image with a D200 and 14" f8 telecscope:
http://www.zodiaclight.com/galleria/...rnPinwheel.htm
Consider, this was done with 30 second individual frames...
Terry