hi,
go to "window manager settings" in the tab "keyboard"
create a new theme, change one of the "custom commands" to "xfdesktop -menu" when asked to press a key, press the menu key. Done.
You can set 10 custom commands, on the contrib site there's a description for using this to change rotation.
BTW : this function is lost in xfce 4.4.beta1 (f**k !!) (it is a stephen king story after all !)
Have fun,
Chero.