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.
So then technically the EF-S lenses with small-image-circle but without short-back-focus should fit on a FF camera (in all their vignetting glory)?
I don't think so... It looks like the lens still has the protruding rubber piece - it's just that the optical design (as opposed to physical design) provides the normal (FF) back focus distance.
I think some third party lenses for crop do fit on FF. I have even seen a picture of it somewhere, I believe it was Sigma
Yeah, all 3rd-party DX/EF-S/Crop lenses should fit on all FF bodies (in theory), because Nikon/Pentax/Sony don't have any special things in the mount that stop you mounting crop lenses on FF bodies (Nikon actively encourage it with their 'DX Crop Mode' on their FF bodies).
Also, seeing as most 3rd-party lenses have to mount on Nikons with their longer-flange distance, the rear element has to be further from the sensor than EFs lenses, so a canon FF mirror shouldn't hit it. They don't redesign their lenses (optically) for different mounts, they just use a thicker mount for canon, thinner for nikon.
I've seen the Samyang 8mm Fisheye mounted on a 5D2, it makes a weird vignetting image because of the hood, you can (carefully) hacksaw the hood off to make it a nice black-edged fisheye (not quite a circle, not quite a FF fisheye) if you don't mind a bit of cropping a lot of people like the effect because Samyang fisheyes are stereographic (compress the borders less).
But you should be able to safely mount any Samyang/Sigma/Tamron/NikonDX/PentaxDA lenses on your FF bodies (should, if you try it and break something don't blame me).