At least on some platforms (e.g. OS X), it is easy to create files with legitimate names containing code points above the BMP limit (<= U+FFFF) currently imposed by Tcl/Tk.  For IDLE 3, I suspect _filename_to_unicode() in EditorWindow could be modified to check for such cases to prevent problems if such file names end up in recent-files.lst.  That might not be sufficient: there may be other problematic places.  I also was able to crash a current IDLE 2.7 just opening a file with such a name.