A winger dribbles the ball in the opposition half, directly down the touchline. He is tackled fairly off the ball by a defender and exits the field of play. The defender passes to a forward teammate who passes straight back; the first defender slips over; the ball continues directly down the touchline. The fallen winger meanwhile, noticing this, sprints down the touchline – still off the field of play – and onto the field to take the ball, alone, and score.
USSF answer (November 20, 2009):
A player who has left the field in the course of play is expected to return as quickly as possible to a position on the field. The legality of the goal would depend on the referee’s perception of two things: Whether this player’s return occurred quickly enough and whether the player stayed off the field accidentally or to deceive the opponents.