iPhoto treats two identical video cameras in totally different ways
Mystery solved!  Behold the following two screenshots.  The first is from Camera 1:

and the second from Camera 2:

See the difference?  One of the cameras had a DCIM folder as a top-level directory (created, I can only assume, when I used that camera once to take a still photo) and the other did not.  Apparently the presence of that folder is what caused the OS to recognize it as a &quot;device&quot;.
I just used Camera 2 to take a single photo, then connected it to my Mac.  It now has a DCIM folder, and iPhoto launched immediately.  That appears to have been it.