The corners of my images are darker than the center at the telephoto end of my zoom's range. Is my ....

Probably not. This effect is called vignetting and it is common in consumer quality lenses. If the effect is not equal in all corners, then something may be misaligned and you should return your camera or lens for service.

Sometimes a lens hood that is not designed specifically for the lens you have can “show” in the picture as the darkened corner.
It is easy to check if this is the case: remove the hood and take another picture in the same conditions and zoom level.

Usually vignetting happens at wide-angles rather than tele-photos. I wonder which lens you are using?

You can also try to counteract this if you have Adobe Photoshop CS2 or CS3 if you shoot in RAW format on the camera. Open the photo in Photoshop's Canon Raw dialog (opening the RAW photo from photoshop should automatically pop up the RAW dialog) and then on one of the tabs you can adjust vignetting either making it darker to add this (some people actually want this look, usually in portraits) but you can move the slider the other way to actually make the corners lighter to try and even the photo out. If you do not know which tab does this, consult the help file.