d) sensors get hot. So hot they ultimately destroy themselves. My D200 has lasted for an hour exposure but the image was entirely unusable and the camera got nearly untouchably hot, but I've heard of a EOS350D which was sent to the repair shop after taking a 40min exposure and cooking the sensor.
I looked in the owner's manual for my XT, it doesn't say. It basically just says that it's going to be noisy and that you might want to use the noise reduction custom function...So is there some number of minutes that you wouldn't recommend going over on a regular basis? I have a XTi