I just picked up a Canon S1 last week.
It's very nice.
My sister has a Canon SD100 the picture quality between the two cameras is for the most part fairly similar when you do NOT need to zoom in much. If you want the big zoom the S1 is your daddy. The IS accually does work. I was playing around taken pictures while shaking the camera slightly and the pics accually come out fairly clear. The other thing I noticed is at max zoom it is hard to keep it 100% steady so the IS is nice for those shots as well.
Anyways back the SD100 it is such a tiny little thing. I was in Vegas last week (where i bought the camera) and it was awesome how my sister could just tuck her camera in her pocket and could carry it to the bar. The S1 is to big to be carrying around drunk and dancing and stuff. So I'd had to leave it behind in the hotel room every night.
So it comes down to, do you want a compact camera you can tuck in your pocket or something you need to carry in a bag or around your neck? I sort of wish I had gone compact, but then again the features of the S1 are awesome & my pictures of those white tigers at the Mirage hotel rock my sis's SD100 because I could zoom in much better. If I was gonna go for a compact camera i'd probably go with the Canon S400 if I was gonna go bigger & bulker the S1. Just decide how you think you will typically use your camera.
The other thing is the S400 is closer to your price range where as I know I paid $499 for the S1. Ohh and while the A70/A75 are awesome (I was considering them as well) do not be fooled into thinking they are "compact" cameras.
Anyways good luck. I love the S1, but when I go drinking partying I know I am gonna wish I had bought a small pocket sized camera.
PS. sorry for long sentances, poor spelling, bad grammer, incoherent ramblings....english is my first language and I failed first year english twice before i passed (3rd times a charm).
EDIT: FIXED SOME BAD GRAMMER