Yes, I know Wikipedia says it stands for short back focus, and lots of other sources do, too (including many of Bryan's reviews). They're all wrong.
The -S in EF-S stands for small image circle.
If you look at the Canon Technical Report for the EF-S 18-55mm f/3.5-5.6 USM, you'll see the notation, "*The "S" in "EF-S" comes from "Small image circle.""
Also, if you look at p. 128 of EF Lens Work III (it's in section 7, The Basics of Interchangeable Lenses...), there's a notation that, "* Some of the EF-S lenses does not employ a short back focus optical system." Yes, that's grammatically incorrect, but correctly quoted, and highlights the fact that while all EF-S lenses have a small image circle, not all of them have a short back focus design (I believe, for example, that the EF-S 17-85mm does not have a short back focus optical design).
Just a little bit o' trivia for you.