are you sure you want to do something like that with mechanical shutter?
Sure, my camera is made to be used!
Are you really sure? At 10-second intervals, you'll have 100000 shots in 277 hours or just over 11 days... and the 400D doesn't even have a *guaranteed* 100000-shot lifetime. I really suggest getting a good compact camera for that (electronic/'half'mechanical shutter, not focalplane-based) but of course, it's your money

This is a concern of mine as well. I wonder if the shutter can be held open (like in "Bulb" position) for the whole shoot and just sample the CCD at intervals. There could be a problem with the CDD overheating, so perhaps the CCD's power could be turned on and off before and after each sample. (This overheating would have to be dealt with for motion detection as well).
I'm just not sure of how the different cameras work internally
(mechanical or electro-mechanical shutters).
We could at least lock the mirror up during the entire shoot.
I know a guy over on dpreview.com who is very knowledgeable about DSLRs and who is excited about CHDK. I'm sure we could consult with him if we need help figuring this out.
CMOS

not CCD.
Anyway, this *might* be possible but only with longer exposures. I.e. the 40D (live view plus 'silent' shooting) can already shoot pictures with only the second shutter curtain, the first is electronic. However, the shutter is still required, at least on short exposures. As far as I know, the chip can't just be 'deactivated' so you'll have to read it while it's still gathering light, causing an uneven exposure across your image. That's why there's still a mechanical shutter, else there wouldn't be any need for one.
Still then, I don't know how the data from the CMOS is transferred to the camera memory (does the OS read it from the CMOS and put it in the RAW buffer, or does the CMOS push it into the RAW buffer with DMA?) so it might be hard to do it in bulb mode. It would be easier to somehow disable the shutter and just take several pictures.