I saw this thing just outside the lens mount ring of my D80 yesterday. Its just above the DOF preview button, it's made of plastic and it can slide back and forth within the narrow hole in the body. Its spring loaded and automatically moves back to its original position.
My Minolta 7000 has this as well but it on the inside of the lens mount(not on the mount itself) so you can't see it while a lens is attached. It's pretty much the same except it's made of metal. I'm quite sure that it controls the aperture on the Minolta because theres another one on the side of the lens which I tried moving it with my finger which then caused the aperture blades to close.
I've never noticed it until now. Can anybody tell me what its meant to do? Thanks.
P.S. Sorry for the lousy picture, it was taken with my phone.