I have an S90 - I find that the ISO shoots up to 800 when I don't want it to be so high as it often results in overblown images. Not to mention the control ring.
My G10 is smarter capping the ISO at 400 but facial features in portraits are not as sharp as S90. I recenlty purchased a GF1, it's pretty smart too capping the ISO at 400. On as side note: I am quite surprised the S90 with IS has less motion blur than GF1 with f1.7 lens. Also the GF1 tend to underexpose in portrait mode under low light - my wife look dark most of the time. Canon tends to brighten skin tone in portrait mode - which is nicer.
I recently found a mode in the S90 that not only caps the ISO at 400 - it also solve the problem of the rear ring from bumping the ISO as well as Exposure compensation.
Use the Portrait in SCN mode!! The rear ring is now in SCN mode instead of ISO or Exposure. And won't mess up the photos! NO more noisy over exposed photos.
I use it all the time now as my automode - now I love S90 after hating it for 2 months after purchase!!
Some comparisions:
1) Easiest camera to use that never goes wrong - G10. Low light results in motion blur. But used with flash is perfect! Macro power with 15MP is number 1. Excellent menu.
2) S90 is wonderful. It is very sharp in low light with it's IS. But the control ring messes up quite a bit and ISO jumps to ISO800 adding noise and overposure.
3) GF1 is extremely smart like G10 too but Canon has slightly better menu control - I have to enter deep into GF1 menu to control flash power - unless someone tells me how. GF1 Underexposes skin tone in low light, my wife looks dark. However I like GF1 for its speed and bokeh. It is an excellent camera but changing lenses is not my cup of tea. I try to stick to the 20mm lens only. Also the Flash recharge cycle is fast. Did you know Canon flash cycle of S90 and G10 is 10 sec? Not acceptable for Prosumer in my book.
Sorry I ended up writing a review here. The thing I wanted to share was how to cap the ISO at 400 in auto mode for the S90 and disable the rear ring from messing up exposures.