One [undoubtedly stupid] question: what does HS stand for?Hybrid Stabilisation.
It is the newest form of Canon's IS technology as introduced by the latest 100L macro lens.
Ummmm...no. You're referring to Hybrid IS (H-IS) in the 100L Macro, which corrects for angular motion like traditional IS, and for translational shift. There is no system in PowerShots that corrects for translational shift.
HS = high-sensitivity, and refers to the combination of sensor technology and Digic image processing that allows for cleaner (relatively, at least) shots in low light conditions.
I think the S100's macro/hybrid IS mode is designed to correct for shift-type camera shake.