I am not using Xinerama. I already have 2 screens working using ATI's fglrxconfig. I don't really want to redo it with Xinerama, particularly not since it is working already, and someone else who tried that ran into minor trouble.
However googling for "fglrxconfig screensaver" lead me to look for the GetViewPortIsFullOfLies setting in .xscreensaver, which caused the single screensaver to cover my full monitor.
So I'm happy now.
Thanks,
Ben