as Hacki posted...thes Ixus models (and also some cheap models of the A-series) have a ND filter instead.
So they don't have the Av menu entries, they have a "ND filter" menu item to set the ND filter
ON (=enabled, swing in - this emulates the highest F-stop, e.g. F/8.0 on the SD870) or
OFF (=disabled, swing out - lowest F-stop, e.g. F/2.8 on SD870),
AUTO means: not controlled by CHDK, the Canon firmware controls it...