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First: there's no chance this will go into 3.1, 3.2, or 3.3 at this point.
Second: I can assure you that switching to the l... version of a function and not following the last symlink is exactly what "follow_symlinks" is for. Please see the 3.3 documentation on follow_symlinks for more:
http://docs.python.org/3.3/library/os.html#follow-symlinks
Third: I think it's better to keep os.isdir() simple. Users who want more sophisticated use cases (what to do about symlinks, using a dir_fd parameter) should use os.stat(). So I'm -1 on this.
TBH I'm not sure this even warrants a documentation change ("for more functionality, use os.stat()").