(a.k.a. How to switch users without entering password, part II) (see part I)

I've got Ubuntu 9.10 with a user account for my wife, and one for myself. "Wake-from-screensaver" should result in "choose user" without having to enter any password. I know how to do that in Windows, but I'm not good with Linux (yet).

Part of this was answered in my earlier question which helped me get past the login screen without passwords (after booting, and after choosing "switch user"), but once the screensaver kicks in and I wake it up again, the system does not present the "choose user" screen. Instead, it either turns off the screensaver and presents the desktop of the most recent user, or (if the screensaver is set to lock the screen) prompt for the user's password (which can be handily surpassed by clicking the "switch user" button and choosing the same user again).

So, the login ("choose user") screen has been dealt with. How do I make the (any) screensaver return to the login screen at wake, rather than to the current user's desktop? Windows can do this, I'm sure Linux can too - but how?