I remember sticking a card in my 30D that my friend had used to take pictures with his Rebel XT, and my camera displayed his pictures perfectly and continued the picture numbering from where his camera left off.
I think the software in these cameras is a lot less fragile than you might think.
I do the same with my 400D (Rebel XTi) and my Powershot A70. The only issue with doing that is the 400D shots show up as very tiny images on the screen of the A70 and you can’t zoom on them or anything. Other than that, the numbering, etc., all works as normal.