I have done some research in order to find a suitable behaviour for
tarfile. I wrote a script to test to what extent all the different tar
implementations transform input pathnames. The results can be found at
http://www.gustaebel.de/lars/tarfile/wwgtd.html.

My conclusion is the following: tarfile now does no pathname
transformation whatsoever except for converting absolute to relative
paths (to stay backwards compatible). This way tarfile is closer to
POSIX, applies less magic and gives more responsibility to the user.

Fixed in r74571 (trunk) and r74573 (py3k). Thanks for your report.