#1. "RE: On a D5000, what for is this hook?" In response to Reply # 0 Sat 30-Jun-12 12:57 PM by aolander
I'm not sure of the official name, but it's the "minimum aperture detection lever/switch". When using a lens with an aperture ring, the switch tells the camera that the lens is properly set to the minimum aperture. (Lenses with an aperture ring must be set and locked at their minimum aperture for metering and aperture control to function. You get the "fee" error otherwise.)
#2. "RE: On a D5000, what for is this hook?" In response to Reply # 1
It appears that any of the dslr's that don't have the AI metering lever have this switch. That includes the D40, D50, D60, D3000 and D5000 series bodies.
What I find interesting is that all manual focus lenses with AI, AIS or factory AI conversion kits also activate this switch when at minimum aperture. However I have not found any Nikon body that makes use of this feature when using a manual focus lens. That includes film bodies as well. So either Nikon has a function they thought of in 1976 and haven't implemented yet or the function they thought of was dropped. I just find that interesting.