javascript:if(document.body.contentEditable=="true"){ document.body.contentEditable="false"; document.designMode="off"; document.body.spellcheck=true;} else { document.body.spellcheck=false; document.body.contentEditable="true"; document.designMode="on";}; void(0);
Shortened version of your bookmarklet, for fun:
javascript:document.designMode=(document.body.spellcheck=document.designMode=='on')?'off':'on'