As noted earlier in this thread, a pre-focussed point-and-shoot also has really low "lag". But it sounds like the 4S really does something well there.

Operationally, I don't know how much of a real-life difference it makes -- literally, I don't know because I've never used the 4S's camera. But I would guess that if I started with the phone in my pocket, and pulled it out for a "snap", the total time taken might be a few seconds longer than doing the same action with a dedicated camera.

That's a form of lag that matters to me, as well as the actual "shutter release" lag.

Cheers