With the recent aquisition of the Canon 40D, I thought I'd put it through it's paces and try some long exposure to see what the noise was like. The results were very pleasing. I'm using a Canon EF 24-70 f2.8L USM lens which is a great lens, although a bit heavy. Click each picture for a larger view.
This first image is a composite of two shots, each 15″ @ f2.8, ISO 400, 24mm focal length. The bright lines at the bottom are caused by aircraft moving through the frame. The break in the line shows the end of one shot and the beginning of the next shot. The stars are just beginning to shift as the Earth rotates.
The second image is a composite of 30 shots over 10 minutes, also east 15″ @ f2.8, ISO 400, 24mm focal length. The stars have a more dramatic shift over 10 minutes. Still, the shift isn't smooth due to the time between images (I need a true intervalometer (Wikipedia) to make this work well). Several aircraft can be seen whizzing through the frame with their stobes flashing regularly.