gnome-keybinding-properties does not show the Brightness Up and Brightness Down keybindings on the OLPC XO.
This is likely because they're not sending the (standardized) key events and instead sending F9/F10 (or similar). See bug 467796
This bug appears to have been reported against 'rawhide' during the Fedora 10 development cycle. Changing version to '10'. More information and reason for this action is here: http://fedoraproject.org/wiki/BugZappers/HouseKeeping
FWIW: Testing build 20090227 from http://dev.laptop.org/~cjb/rawhide-xo/ on an XO (us keyboard layout) brightness keys are still having no effect.
gnome-keybinding-properties doesn't show brightness up/down for anyone. If you want the brightness up/down keys to have an effect, make sure they're bound to XF86XK_MonBrightnessUp/Down for gnome-power-manager to pick them up.