Created attachment 4782 
a screenshot of my desktop after restarting X

ArchLinux, x64
Linux di 3.6.8-pf-di #1 SMP PREEMPT Fri Nov 9 04:49:15 EET 2012 x86_64 GNU/Linux

virtual display is 1600x1600 and real resolution is 1280x1024

# the Screen section of xorg.conf

Section "Screen"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    Option         "TwinView" "0"
    Option         "Stereo" "0"
    Option         "nvidiaXineramaInfoOrder" "CRT-1"
    Option         "metamodes" "1280x1024 @1600x1600 +0+0"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection

When changing virtual desktop on the fly by nvidia-settings, xfce won`t display nothing at all, cutting the panel by pixels going outside of display resolution.

After restart X the wallpaper won`t be displayed out of display dimensions, all the menus and tips are displayed *inside* display resolution area when clicking on oustide, but the panel is displayed correctly at virtual desktop edge. Also desktop icons are displayed correctly at the whole desktop.

Look at screenshot, pls.