My thesis is that Chell is a prisoner.
1. She wears an orange body suit, much like that in the real prison.
2. GLaDOS states that she is "not a good person". Yes I know GLaDOS is bat shit insane, but still she was true about several things.
3. Chells face is far rougher than that of any other Source model I've seen. Her face is rugged and has a look of disgust on it, as if she has been toughened by something, such as prison.
4. GLaDOS said that she "You’re not smart. You’re not a scientist. You’re not a doctor. You’re not even a full-time employee," So she may possibly be a volunteer prisoner, sweeping trash and what not. And it would be easy for a prisoner to become a test subject as not many would notice or possibly care.
5. The personality trait of ingenuity. She is a born escape artist, a possible trait of a thief. I know this one is a stretch.
(sorry if this is #2, missed the warning) agree with the possibility of Chell being a prisoner, as she was basically imprisoned in the Aperture Science facility anyway.