I never had any problems with my original 5D, but I’ve heard of the mirror falling out in some cases. Perhaps there was some sort of problem with the attachment mechanism? Well, I’ve noticed that on the new 5D Mark II you can see two metal tabs that appear to be holding the mirror in place. Take a look at the comparison pictures. You can click for larger versions.

Canon 5D	Canon 5D Mark II (tabs are circled)

Maybe this will reassure any original 5D owners who had mirror problems.