The impetus question is what was throwing us. The NFL rules digest has the following definitions:
14. Impetus: The action of a player that gives momentum to the ball.
30. Touchback: When a ball is dead on or behind a teams own goal line, provided the impetus came from an opponent and provided it is not a touchdown or a missed field goal.
Safety
1. The important factor in a safety is impetus. Two points are scored for the opposing team when the ball is dead on or behind a teams own goal line if the impetus came from a player on that team.
(a) Blocked punt goes out of kicking teams end zone. Impetus was provided by punting team. The block only changes direction of ball, not impetus.
This last would seem to imply that a player on the return team cannot provide the impetus.
http://www.nfl.com/fans/rules/
[Edited by coogrfan on Aug 11th, 2005 at 03:28 PM]