omg, same thing happened to me about a month and a half ago. I just never posted on ATOT about it. 😀
I was in the same position (backing up), and after inspection, we both agreed not to do anything about it because the damages were minimal (I had a broken taillight, and she had a dented bumper), we both decided not to get insurance involved.
About a month later, I got a letter from their insurance company saying that I was at total fault and owed $1300. :Q
I went to my insurance company, and now they're currently investigating.
Moral: Always exchange information no matter how small the damage is, and if both of you agree not to do anything, get it in writing.