I have a A470 and with the ISO down to 10, exposure time to 10 seconds, and ND filter on, my pictures still come out overexposed. Any help would be much appreciated.
In general, CHDK ISO overrides do not work much beyond the stock canon range. So even if you enter 10 in the UI, you are probably getting around 80.
To get longer exposures in bright light, you would need to add an additional external ND filter.