Patch #683592 revisited, after discussions with MvL:
- Implement the behavior as specified in PEP 277, meaning os.listdir()
  will only return unicode strings if it is _called_ with a unicode
  argument.
- And then return only unicode, don't attempt to convert to ASCII.
- Don't switch on Py_FileSystemDefaultEncoding, but simply use the
  default encoding if Py_FileSystemDefaultEncoding is NULL. This means
  os.listdir() can now raise UnicodeDecodeError if the default encoding
  can't represent the directory entry. (This seems better than silcencing
  the error and fall back to a byte string.)
- Attempted to decribe the above in Doc/lib/libos.tex.
- Reworded the Misc/NEWS items to reflect the current situation.

This checkin also fixes bug #696261, which was due to os.listdir() not
using Py_FileSystemDefaultEncoding, like all file system calls are
supposed to.
diff --git a/Misc/NEWS b/Misc/NEWS
index bb13310..5bf335a 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -41,11 +41,9 @@
   about the rationale is in the NEWS entry for that.  See also SF bug
   report <http://www.python.org/sf/693121>.
 
-- os.listdir() now returns Unicode strings on platforms that set
-  Py_FileSystemDefaultEncoding, for file names that are not representable
-  in ASCII.  (This currently only affects MacOS X; on Windows versions
-  with wide file name support os.listdir() already returned Unicode
-  strings.)
+- On Unix platforms, if os.listdir() is called with a Unicode argument,
+  it now returns Unicode strings.  (This behavior was added earlier
+  to the Windows NT/2k/XP version of os.listdir().)
 
 - Distutils: both 'py_modules' and 'packages' keywords can now be specified
   in core.setup().  Previously you could supply one or the other, but
@@ -88,8 +86,8 @@
 Mac
 ---
 
-- os.listdir() now may return Unicode strings on MacOS X. See the general
-  news item under "Library".
+- os.listdir() now returns Unicode strings on MacOS X when called with
+  a Unicode argument. See the general news item under "Library".
 
 - A new method MacOS.WMAvailable() returns true if it is safe to access
   the window manager, false otherwise.