"Under graphical environments (such as Gnome or KDE) 'Alt'+'PrintScrn/SysRq'+key combination generally only leads to a screenshot to be dumped. To avoid this Print Screen feature the magic SysRq combination should be additionally prefixed by Ctrl, becoming 'Ctrl'+'Alt'+'SysRq'+key. "
Whilst I agree that it's undesirable to have two parts of the system fight for the same keypress, it seems this is well known. From wikipedia:-
http:// en.wikipedia. org/wiki/ Magic_SysRq_ key
"Under graphical environments (such as Gnome or KDE) 'Alt'+' PrintScrn/ SysRq'+ key combination generally only leads to a screenshot to be dumped. To avoid this Print Screen feature the magic SysRq combination should be additionally prefixed by Ctrl, becoming 'Ctrl'+ 'Alt'+' SysRq'+ key. "