Of course it isn't 50mm though when it is mountedon the body. It is a little bit more.
Nitpick: actually, it
is 50mm when it's mounted on a 400D (XTi, whatever, I'm an Australian, my tendency is to talk about the Australian model names.
) It doesn't matter whether you have a 50mm lens for a large format camera, medium format camera, 35mm film camera, full frame digital body, 1.3 crop digital body, 1.6 crop digital body, or even a digital compact - it is still a 50mm lens regardless.
What differs is the field of view. The smaller sensor of the 400D means the image created by the 50mm lens is cropped to the centre portion. If you took a picture with the 400D and the 50mm lens, and then took that exact same picture with the exact same lens mounted on the 1D mark 3, and then on the 1Ds mark 2, you'd see that the image created by the 400D is the same as the 1D mark 3's image cropped a little, and the same as the 1Ds image cropped a lot. (I'm ignoring the pixel count for the purpose of this discussion.)
Don't confuse "crop factor" with "focal length multiplier" - the two are not synonymous, and there is a difference between a 50mm f/1.8 on a 1.6 crop body and an 85mm f/1.8 on a full frame body - the latter will have a shallower depth of field, in particular. (It's also a slightly narrower field of view, but not enough to worry about.)
