Fixed in r64499 (trunk) and r64500 (py3k).

I now get:
>>> import locale
>>> locale.setlocale(locale.LC_ALL, "en_US.UTF-8")
'en_US.UTF-8'
>>> for x in
(123,1234,12345,123456,1234567,12345678,123456789,1234567890,12345678900):
...  print("[{0:>20n}]".format(x))
... 
[                 123]
[               1,234]
[              12,345]
[             123,456]
[           1,234,567]
[          12,345,678]
[         123,456,789]
[       1,234,567,890]
[      12,345,678,900]

and:

>>> for x in
(123,1234,12345,123456,1234567,12345678,123456789,1234567890,12345678900):
...  print("[{0:>10n}]".format(x))
... 
[       123]
[     1,234]
[    12,345]
[   123,456]
[ 1,234,567]
[12,345,678]
[123,456,789]
[1,234,567,890]
[12,345,678,900]