In the 2.x branch tarfile is not prepared to deal with unicode pathnames at all. This changed in Python 3. The fact that it works anyway (in the majority of cases) to add filenames as unicode objects is pure coincidence - I suppose you have a utf-8 system encoding. On a latin-1 system your script would fail much earlier during the add() call.

Some reading: http://docs.python.org/library/tarfile.html#unicode-issues