All,
The ARISS L-band/S-band antenna systems are Left Hand Circularly Polarized. There are some good details of the antenna systems in this paper:
http://www.amsat.org/amsat/ariss/Papers/Phase%202%20AGM03Final.pdf
And this presentation:
http://www.amsat.org/amsat/ariss/Presentations/AMSAT%20Symposiums/AMSAT03.pd f
Chart 25 of the above presentation shows the antenna locations looking down the axis of the Service Module. In this picture, the ISS velocity vector is either in or out of the page and Earth is towards the bottom of the page. This assumes they are in the normal Earth pointing mode (which is not always a given).
We have some good on-orbit views of the antennas in this set of charts too. Enjoy!
I hope this helps. And thanks for all the interest in ARISS.
73, Frank, KA3HDO