According to most American sources (e.g., The Chicago Manual of Style) and to some British sources (e.g., The Oxford Guide to Style), an em dash should always be set closed (not surrounded by spaces). But the practice in many parts of the English-speaking world, also the style recommended by The New York Times Manual of Style and Usage, sets it open (separates it from its surrounding words by using spaces or hair spaces (U+200A)) when it is being used parenthetically. Some writers, finding the em dash unappealingly long, prefer to use an open-set en dash. This "space, en dash, space" sequence is also the predominant style in German and French typography.So while I don't know where you are, the most common American usage would be "The adept poster—one of nobility, wisdom, and wit—is the superior MeFite."
posted by Inspector.Gadget at 8:52 AM on September 2, 2008