Rosebud.

IIRC, and correct me if I'm wrong, it's not really an "s" per se, but a glyph. In the beginning of the first Superman movie when Jor-El was at council telling of his findings on Krypton's soon to be fate, all the council members had different symbols on their chests... Jor El's just happenned to look like an S.

That's at least what I take from the movie version.

I also like how his costume is supposedly made from the cloths he was draped in when placed into the spaceship before his depparture from Krypton.

That was sort of my instinct when I thought about it. I can picture the costume designers wondering how they could make sure everyone knew that Jor-El was Superman's father.

Let's put an S on his shirt!

The real answer is that in the Superman comic book the "S" was never used until his costume was made (which I believe was never shown until much later. In fact, Jor-El in the origianl comic had no lettering just a shape on his costume/clothing.

In the movie, Mario Puzo used these "symbols" to refer to a specific family heritage, which is why all the council members have different symbols. This "movie" version of the history of Superman has now been added to the Superman "legend".

to see the 30's and 40's comic book versions of the Superman origin go here.

Also, go herefor a history of the costume itself.