Originally posted by smomofo
It seems to me that the en passant rule exists so that one can not use the two-space first move as a way to slip a pawn past an opponent's without the opponent at least having the option of capture.
Yes,this is the intention. When chess was invented the Pawns could only move one square on any move. When the double square option was introduced for any Pawn's first move, it was done simply to speed up the early part of the game, and it was decided that the chance for the opponent to make a capture as if the Pawn had only moved one square should be available. This is also why the e.p. capture must be made immediately, as obviously the endangered Pawn could have moved to it's fourth rank if not captured at once.