Eagles :
My understanding from the sheer physics point of view is that the speed is irrelevant.
If you bunt ( push over ) at any speed you're risking low G. Other possibilities I'm aware of are at the top of a cyclic climb, again if you push over in an ill-advised plan to attempt to continue flying, rather than planning the finish of the climb well ahead to keep G positive.
[This message has been edited by WhoNeedsRunways (edited 03 November 2000).]