It can be difficult to find... as it is hidden some of the time.
On a new SD990IS/IXUS980IS, I just spent half an hour going through the camera menus, searching on this forum, going through the CHDK menus and finally found PhyrePhoXs post above. (I set this on my SD1100IS when I first started using it 6 months ago and promptly forgot how to do it)
On my new cam, REVIEW does not show up unless the camera has the lens extended and is in shooting mode as opposed to playback mode. The SD990IS powers up with the lens retracted. Looking through the menu system just will not show the REVIEW function. Doh. Finally, I noticed this behavior and solved my problem.