Having owned quite a few Canon DSLR cameras made with magnesium construction, I am pretty familiar with the overall build quality. I consider the build of most of their mid-range to high-end cameras to be superb. The magnesium construction and weather sealing is really what separates the Canon Rebel line of cameras with everything above it. A camera will never be waterproof so the best you can hope for is that it will keep the water out in moderate to heavy rain and snow.
As part of this discussion, I think it's worth talking about how a camera is going to end up getting treated by the end user. If you are using someone else's equipment, then I suppose most people wouldn't object to taking the camera out into extreme environments (much like when you are driving a rental car) with little regard. I have spoken to a few of the local newspaper photographers in my area and looked at their equipment. They check their cameras and lenses out before heading out to cover a story. The equipment looks totally trashed. It makes me cringe!
I would never deliberately bang or drop any of my cameras. I would also never deliberately get them wet. I carry a large, heavy duty zip-lock bag with a hole cut out at the end (for the lens). I always slip this on the camera I am using if it's going to be exposed to moisture for more than a few minutes. (A zip-lock back might sound cheesy, but it works very well and it folds up and stores much better than an expensive, commercial product that does the same thing.)
I have never completely submerged any of my cameras, but they have all been wet in one way or another multiple times. The most common issue that comes up (for me) is when a light rain storm rolls through during an environmental portrait session. I never had any issues with the exterior of my cameras getting moderately wet. Of course, I never change lenses or expose the sensor if I have a wet camera.
I think the bottom line is to remember that a modern day DSLR (whether a low end Canon Rebel or a high end 1D) is filled with sophisticated electronics. Most of the electronics are exposed to the outside world the second you take the lens off. I think you have to treat you cameras carefully - irregardless of whether it's raining or not.
Finally, I have never submerged my 7D or even had it substantially wet, but I will say that the build quality is superb. It's an amazing camera for the price.