The “⌘” symbol (the “Looped square”) was chosen after Steve Jobs decided that the use of the Apple logo in the menu system (where the keyboard shortcuts are displayed) would be an over-use of the logo. Apple’s adaptation of the symbol—encoded in Unicode (and HTML) at U+2318 ⌘ (HTML: ⌘)—was derived in part from its use in Scandinavian countries to denote places of interest. The symbol is known by various other names, including “Saint John’s Arms” and “Bowen knot”.
[via Wikipedia. http://en.wikipedia.org/wiki/Command_key]