This thread has several bits of inaccurate information sprinkled throughout. It is NOT only the "buckhorns" or the "rabbit ears" that may interfere with the mounting of pre-AI lenses on the non-professional Nikon bodies--it may also be the diameter of the aperture control ring in and of itself, hitting the minimum aperture sensing tab on newer bodies that do not have an Ai-indexing tab.
The aperture rings on many pre-Ai Nikkor lenses are "fat", ie. large in diameter, and removing the buckhorns will NOT make the issue on non-compatibility go away simply by removing the buckhorns. The entire aperture control ring on most pre-AI lenses will hit the minimum aperture-sensing pin, located at the 7 o'clock position on the front of the camera on most, but not all, newer Nikon bodies. The Ai-indexing follower tab is located at the 1 o'clock position,and it too can be hit and damaged by the large-diameter of the aperture control ring of a pre-AI lens, even if the buckhorns have been removed.
The Nikon D40, D40x, and D60,as well as the D3000 and D5000 ALL lack both the have the minimum aperture sensing pin and the AI indexing follower, and as such, will accept pre-AI, AI, Ai-S manual focusing lenses in Nikon F mount. Repeat this over and over--the ONLY Nikon bodies that can mount pre-AI lenses with no modifications are "the five baby Nikons".
As to the contention that pre-AI lenses will go "too far into the body"--that contention is not acurate. There are fewer than a handful of Nikon lens designs that protrude far into the body, and ALL of them are very rare, highly-collectible Nikkors made in the 1960's and early 1970's, like the 21mm f/4 mirror lockup lens, and a couple of exotic old-school fisheyes that required mirror lock-up. Fewer than five lens designs "protrude into the body"--that is an issue Canon d-slrs have with some aftermarket lenses, and is simply not applicable to the over 50 million Nikon F-mount lenses Nikon itself has made, as well as millions of 3rd party F-mount lenses.
Lately, I have seen a couple of specific posters claiming that "any Nikon lens will fit" on any Nikon body. That is not true! ONLY the five entry level models, D40, D40x, D60,and D3000 and D5000 which lack BOTH a 7 o'clock minimum aperture sensing tab and which lack the Ai indexing follower at 1 o'clock--only those five bodies, can use all Nikon lenses from 1959, except for the rare "invasive fisheyes".
Here's a chart for reference.
Nikon Lens Compatibility