That's what people THINK it meant. I don't think it's ever been stated by Apple or SJ.
Note in this article the author says "that's what the 'S' REALLY means". My take on that statement says that we still don't know what it means, just what we THINK it means...or what the general public has accepted as the meaning. That's why I claim no idea on it's meaning.
The "S" in iPhone 4 S has never been explained by Apple.
I could swear during the iPhone 3GS special event the [S] was stated as being for speed. However I don't recall any such statement for the 4S though it stands to reason that I my first sentence is accurate then the latter likely is too.
That said it's quite possible with so many [S] designations to append this time around a good marketing move would be to leave it up in the air. To never officially define it.