Vesbot,
Yes we are just talking semantics.
The magnitude of the ground velocity of the Cessna is 10 knots, its direction being 180 degrees to its heading.
Its position over the grond is changing by 10 nautical miles every hour, and there is no such thing as a negative nautical mile.
I would maintain speed is a scalar quantity, and therefore has no direction. By invoking a positive or negative you are now actually defining a reference frame, and we are talking velocity, not speed.
I think our passengers actually demonstrate this- how can you change speed without accelerating?