One I hear is that she was the daughter of scientist. This theory arises from the fact early in the game, GLaDos mocks you saying, "Bring your daughter to work day is a fine time to get her tested." I personally am against this thought simply because of how early it is in the game. GLaDos lies to you throughout the story, with her biggest lies being at the beginning. She even goes out of her way to tell you she was lying. She promises to stop lying, but never does.
Another interesting Chell theory is that she is an Android. This would make a bit more sense to me. She is in a world where no person is, only machines. She doesn't speak or even show any real reactions to anything GLaDos says or any of the mind bending science. And several times within the game she calls you an Android, she tells you Android Hell is a real place you will go to.
The strength of either argument requires quite a bit of trust in the character of GLaDos. Which, as I stated, she lies almost nonstop. I currently haven't beaten Portal 2 yet, so There might be a huge reveal showing that you are a back up program of GLaDos downloaded into a comatose human body. After all, some of the Theories stated seem just as ludicrous.