what does the 'S' mean?

Nope. After the 3G came out, the 3GS was produced. Although Apple never said what the S stood for, it does not stand for Steve or Siri. It could definitely be speed or super though. It has a lot to do with minor improvements.

I belive that the "S" means "Super".

Apple never really said - there are lots of guesses, though 🙂

It may not stand for anything.

S stands for Steve. iPhone 4 Steve to pay tribute to him.

I believe it stands for speed

http://www.youtube.com/watch?v=q2RuEuAuloA&feature=youtube_gdata_player

I think the "S" stands for "special".