I see a lot of threads if forums (both astronomy and photography) comparing the different versions of the Canon 200 lenses. I was doing an update on my website today and noticed I had a comparsion between the 200 f2.8 prime and 70-200 f2.8 non-IS zoom at this URL:
http://www.pbase.com/terrylovejoy/200_comparision
In my opinion the prime gives somewhat more contrast and pleasing star images at the image centre, but the zoom gives significantly better images towards the edges. I still think it is great result for a zoom though! I've also updated my 70-200 page with shots of the Crux at 70mm-200mm. These are just 75 second exposures with an unmodified 350D from backyard in Brisbane.
http://www.pbase.com/terrylovejoy/70_200
Terry