The manual warns that you should turn the camera off to remove the lens, but it doesn't say why.
I suspect this may have to do with dust, but I could imagine there are lots of other reasons. I'm hoping someone can tell me the reason for doing this, so it will help me to never forget :-).
Answer
When I get this question from my students, the underlying question is often based on the persistent myth that taking a lens off without first turning off the camera will 'fry' the lens, the camera, or both (depending on who you talk to).
With both Canon and Nikon cameras the leftmost pins on the body (when looking at the camera) are the VBat (6 volt lens power) pins, so when the lens is disengaged and rotated counterclockwise the first thing that happens is that power is removed from the lens. Since none of the other lens contacts sweep over the VBat pin, that scenario is actually physically impossible. Additionally, since the lens power is disengaged from the moment the lens is rotated, the lens is no longer able to pass information back through the communication (dcl and dlc) and clock (dclk) pins. Thus there is very little danger of bad data getting passed between the camera and the lens and somehow 'corrupting' anything unless you really snap the rotation of the lens and there "happened" to be communication going on at just the right moment. In any case, simply turning off the camera and turning it back on would clear such an error.
While I have no direct experience with the pin and contact patterns of other brands of SLR/dSLR, I would be very surprised if the others (Sony, Pentax, etc.) did it differently.
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