While doing some camera tests I took a few interesting astrophotography images using a Canon 500/4.5L lens. The first isn't too exciting, but it's a rather nice shot of the moon taken using the 500mm lens with a 2x multiplier. I used "live view" to optimize focus and I used a remote release and mirror lockup (though for the moon image that probably wasn't necessary).
The image above shows the full frame (well, the full APS-C frame of the 40D), so you can see that with a 1000mm lens you get a pretty decent sized image of the moon. Below is a cropped version.
While shooting the moon I noticed that Jupiter wasn't far away from it, so just for an experiment I swung the lens around so that Jupiter was in the center of the frame and took a few shots. I wasn't expecting much other than a bright blur. When I looked at the image I was quite surprised to actually see some detail on the planet's surface. The horizontal bands are real. Even the color is approximately correct!
While the image certainly won't win any awards, it's still better than I would have expected. With a 1/8s exposure I got the image below. Jupiter is greatly over exposed of course, but the three dots to the left are actually 3 of the moons of Jupiter. Reading right to left they are Io, Ganymede and Callisto. Europa (the other bright moon) was crossing the disk of Jupiter at the time of exposure, so isn't visible. Note that with a 1/8s exposure, the images trail slightly due to the motion caused by the earth's rotation.