Recently sold a 17-85mm EF-S lens - I felt that it was great as a daytime walk around lens (great FL range), but not so good as an astro lens - bit heavy on the distortions.
I have the f4 version of the Canon 70-200mm L lens, very sharp for day and night use. Sharp stars right to the edge of the field. If the lens is going to be used mainly for astro work, save some money and get the non IS version.
The L lenses are superior for astro imaging IMHO. The f4 70-200 is thought to be as sharp as the f2.8 lens, but as you say the aperture difference might swing it for you (67mm v's 77mm). Either way the 70-200mm FL range is great for wide field imaging (not so much 'wide angle'). Attach a 1.4X extender and you've got a very nice 280mm lens. The 2X extender doesn't get such a good rep though!
The f2.8 is a much larger beast than the f4 lens - so a solid method of mounting the larger lens must be factored in too.
Personally I would avoid the 17-85 for astro imaging. Some reviews show the 17-40 L to suffer from barrel distortions - but others may like to comment on that lens. Here's a couple of review links...
http://www.photozone.de/Reviews/Cano...report--review
http://www.photozone.de/Reviews/Cano...report--review
Good luck
Doug
ps...in the 40D signature link below, the Large Magellanic Cloud pic was taken with the 17-85mm lens and the last 2 pics - Southern Cross and Widefield Eta Carina were taken with the 70-200mm f4L. Comparison may help.