Well, it certainly makes sense that they would share a good deal of hardware, minus the obvious features such as the pentaprism and magnesium alloy body and that kind of thing. But the sensor, focus system, and probably much of the electronics are the same. This isn't unusual, and I'm still surprised that some people find it upsetting that any manufacturer would "defeature" a lower-priced product. That's the norm, not the exception.
When you buy a car, it comes prewired for just about every option that is available on that model whether you buy that option or not. Everything is designed to make it as universal as possible since this is the most cost-effective way of building them. Once you buy that car, you can use that wiring to add your own fog lights, auto-dimming mirrors, or whatever feature you want with aftermarket products. I doubt if its that much different for cameras.
Granted, modular pieces like memory chips or other things that can easily be added or subtracted may be left out on the less expensive model, but I'm pretty sure that Canon would try to keep the unique stuff down to a minimum.
So yes, perhaps a software solution for gaining some 10D features on the Digital Rebel is probably possible. And I don't worry too much about anybody making such a change to their own camera, or discussing such changes freely so long as nobody is stealing Canon's patented knowledge for commercial purposes. I mean, its their camera - they can do what they please with it.