This seems to have something to do with the current locale.
On OS X 10.4.11/PPC I have:

$ echo $LANG
C

and the test fails.  On OS X 10.5.5:

$ echo $LANG
en_GB.UTF-8

and test_cmd_line.py passes.  Moreover, after doing:

$ export LANG=C

test_cmd_line.py fails on OS X 10.5 too in the same way.