I have never sent my camera in for Preventive Maintenance. I tend to prefer the 'if it ain't broken don't fix it' method of camera maintenance.

One item that needs constant maintenance is removing dust on the sensor. The 40D, with its sensor dust removal shaking is better than most, but eventually, it needs to be cleaned. From what I have read from others, often sending in to Canon results in no improvement in sensor cleaning, or it being worse. At one time it was conjectured that Canon merely blew air on the sensor as 'cleaning'. For this reason, I do not send in my camera for this service, but perform it myself using the easy, and safe Copper Hill method.

Cameras are generally built for their purpose, especially your 40D. The parts that typically 'wear out' are the shutter box mechanism, which, due to the complexity and expense, are rated for a certain expected life in number of shots. This life is not dependent on regular maintenance (you don't do oil changes etc).Some of Canon's cameras actually have a stated shutter life, but I don't think they published one for the 40D,though rumors suggest 100k shots. Whether yours will last longer is a roll of the dice, but sending your camera in won't extend the life. Given the replacement cost (~$300+), waiting for failure is the best policy.

If you have problems with achieving sharp focus with a lens, sending in the body and lens for calibration is highly recommended, and generally I have found much better focus performance as a result. Whether this is because they fixed something, or because it is psychological on my part is hard to tell, but I am satisfied.

While it is possible that, like cars, there are some internal service bulletins that would provide an unknown fix you didn't ask for, I suspect that those are to ensure that the camera does not come back to be fixed for that issue. I would prefer to wait for that issue to arise.