Not reproduced with Inkscape 0.48.2 on Mac OS X 10.5.8 (i386) - using custom shortcut to compensate for loacized keyboard where '|' can't be created without additional modifier: guides can be shown and hidden with both the menu command and the keyboard shortcut, independent of window/fullscreen (internal)/maximized (window manager) mode.
> Trying by Menu or | shortcut, but no action.
Not reproduced with Inkscape 0.48.2 on Mac OS X 10.5.8 (i386) - using custom shortcut to compensate for loacized keyboard where '|' can't be created without additional modifier: guides can be shown and hidden with both the menu command and the keyboard shortcut, independent of window/fullscreen (internal) /maximized (window manager) mode.