F17: Could not find working solution to set virtual desktop size

A have netbook with screen 1024x600 and would like to have virtual desktop with virtual 1280x1024 size
xrandr --fb 1280x1024 --mode 1024x600 --output LVDS1 --panning 1280x1024
sets desktop size to 1280x1024 but mouse is locked in 1024x600 viewport
I have tried add "Virtual 1280 1024" option to xorg.conf but this does not help