Here is a link to long exposures for the Canon 5D & some 30D info.
http://photo.net/canon-eos-digital-camera-forum/00QWU0
With long exposures, the sensor is on and charged for the full exposure time (of course). This heats up the sensor, and causes many of the photosites to react to the heat of the neighboring photosites. This reaction is recorded as a color, in the processor. This is digital noise. The sensor tends to react predictably, and will create almost the same noise, in the very next frame.
With that knowledge, the camera companies created a noise reduction process called, Dark Slide (a term from back in the film days I believe). Here is how it works.
1) You make a long exposure (say 30 seconds).
2) When the exposure is complete, the camera leaves the mirror down, and fires up the sensor for another 30 seconds, to finish the total exposure process.
3) The processor takes all of the color noise that showed up on the 2nd frame (dark slide), and subtracts it from your original exposure. This removes most of the color noise from your image.
Good or bad, that is how it works. I know the cameras use this process in the 15 second to 2 minute range. I don't know if they do that for a 1 hour (or longer) exposure. Batteries might be dead before that could all finish up.
Also: The Dark Slide setting can be turned on/off in all the cameras I have seen, that offer this function.