It's kind of funny you mention that because as I reading through this thread and reading Tom's comments about parthenogenesis being rare in snakes (which it is.) I was thinking I had heard something about some vipers having parthenogenic births and that was it.
He did a talk a few years back for cflhs and mentioned that birth. I believe Jim Peters made some references to a few other cases of this also involving vipers. It was a few years ago though so my memory is a bit hazy on it.