The Nikon D80 camera contains a EXIF tag value that counts the number shots taken
|Number of shots taken by camera
This can be extracted by running the following command
exiv2 -pt picturename.JPG ... Exif.Nikon3.ShutterCount Long 1 5263
This example shows that there have been 5263 shots taken.
I guess this EXIF tag exists on Nikon D40x, D60 and D70 also, but this is not verified.
More Nikon specific EXIF tags can be found on http://www.exiv2.org/tags-nikon.html