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Hmm. When the string comes from sys.argv, it should be in
the user's preferred encoding, not in the file system
encoding, which would suggest that the current code is right.

When the string comes from tk_getOpenFile, I would expect to
get a Unicode string back instead of a byte string. I can
believe that Tk fails for OSX here: it relies on Tcl's glob
command, which apparently assumes that "encoding system" is
used for the file system; try

 >>> [unicode(x) for x in t.tk.call(('glob','*'))]

There are more issues OSX glob, e.g. for Latin characters,
it processes the decomposed form inconveniently, see

http://sourceforge.net/tracker/?func=detail&aid=823330&group_id=10894&atid=110894

So I think it is fine to display question marks on OSX if
necessary;in general, it now seems that the locale's
encoding should be used indeed.