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I could be totally off the mark, but I'm pretty sure the Canon front (dust shaker) filter is for antialiasing, almost like a matte glass, designed to blur the image just enough so that a beam of light falls on more than one pixel in the bayer matrix. This antialiasing filter should prevent rainbow and moire effects in everyday use. Probably not required for astrophotography, where sharpness is very important. Looking at Christian's report, this glass also has a gentle UV/IR curve, probably insufficient for astro, so fitting an external UV/IR filter, as Fred has done, is a good thing to do.
The whole set-up of the 40d filter stack makes me wonder if Canon are planning a 40da?
Cheers,
Rob
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