I inherited my Nikon D40 from my grandfather. When I first tried to use it, the main battery was flat, and I concluded in retrospect that the internal battery (what I would call the CMOS battery in a desktop computer) must also have been flat: the date needed setting, and the first time I set it the settings were then lost. The second time, after the charged battery had been sitting in the camera for a while, the settings were preserved.

When I heard that shutter activation number is included in the EXIF I had a look, and the number corresponded with the number of shots I'd taken. So there are two possibilities: either my grandfather had never used the camera at all (which I find unlikely, given that he'd owned it for seven years and bought two lenses in addition to the kit lens) or the shutter count is stored with the other settings in semi-volatile memory, and can be reset by leaving the camera without a battery for a long time. The camera had been sitting around unused for at least a year before I charged the battery.