Maybe os.path.supports_unicode_filenames should be deprecated.
The doc currently says:
"True if arbitrary Unicode strings can be used as file names (within limitations imposed by the file system), and if os.listdir() returns Unicode strings for a Unicode argument."

On Linux both the things work, even if the value of os.path.supports_unicode_filenames is still False:
>>> os.path.supports_unicode_filenames
False
>>> open(u'fòòbàr', 'w')
<open file u'f\xf2\xf2b\xe0r', mode 'w' at 0x9470778>
>>> os.listdir(u'.')
[u'f\xf2\xf2b\xe0r', ...]
>>> open(u'fòòbàr')
<open file u'f\xf2\xf2b\xe0r', mode 'r' at 0x9470778>