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I have a 17-85 and regret getting it... it's very soft (not sharp), slow (aperture) and often misses the focus. I was tossing up between that, the 17-40 L and the 18 - 55 IS f2.8 when I got the camera and I should have got the 17-40 or the 18-55 2.8. Another option is the Sigma 18-50 f2.8 macro - but not the HSM version. A few people have said the non-hsm is sharper than the hsm version. If you look around, this lens is about $500. I'm still considering the 17-40, particularly as I can also use it on my EOS3.
But if you want to take photos without tracking, you are limited to less than maybe 30 or even 15 seconds if you want to minimise star trails. This implies a fast lens - like the 50 mm f1.8 (very cheap, good optics, crap construction), or the 50 mm f1.4 (expensive, good optics, good construction). A 50 on a 40D will be slightly telephoto. If you want 'standard', then the 28 f1.8 is a good choice. If you want slightly more telephoto, the 85 mm f1.8 is a great lens. Anything more than that then you will need tracking.
I've also got the 70-200 f2.8 and that is a really fantastic lens, although very heavy.
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