I just found this interesting youtube vid & facebook item and thought I had to share it here.
See:
Youtube video VW bug in a wind tunnel
And there was a poll on the face book page, see the top red box. The answer is in the bottom red box:
(click on it to get to the entire facebook page, I couldn't link directly to this item)
The beetle has 6% less drag when going backward.
I think it's because of attached flow staying attached to the rear window, which creates drag. Going backwards, there's nothing to attach to after the windscreen point. So the airflow can form itself like a teardrop.