ONLY the preview is reversed in Photo Booth (and some other apps). This is by design. We are all so accustomed to seeing ourselves in the mirror that your own image would look strange to you if it were the other way. For this same reason, iChat and some other apps where we see ourselves work like Photo Booth. Remember, only the preview is reversed. All output will be left-right correct.
You can test what I say by making a Photo Booth picture of a book cover or some other large text. The preview will be reversed, but when you click the finished picture, it will show, email, print, etc., right-way-round.
Note that not all apps work the same way in this regard. iMovie, QuickTimeX, and some other apps do not reverse their preview windows.