See http://svn.boost.org/svn/boost/sandbox/filesystem-v3/libs/filesystem/doc/v3_... for a fuller description of the prototype and links to the prototype implementation.
locale( "" ) is not correct on Mac OS X, where the API always takes UTF-8 char* and the filesystem uses wchar_t internally (AFAIK).
This is irrelevant from a practical point of view, but the standard requires that "" be recognised as a valid locale: "explicit locale(const char* std_name ); Effects: Constructs a locale using standard C locale names, e.g. "POSIX". The resulting locale implements semantics defined to be associated with that name. Throws: runtime_error if the argument is not valid, or is null. Remarks: The set of valid string argument values is "C", "", and any implementation-defined values." John.