I moved in with my girlfriend/fiancee pretty about 6 months after finishing highschool. I moved in with her and her best friend Jacqueline and lived there for just over 2 years. My parents were pretty cool with it all, but my parents are very liberal in their approach to life, not very restrictive at all. Their only concerns were the financial sides of things but all that was sorted and they were fine after that. Dad was pretty impressed with the whole situation seeing me be independant and living with my girlfriend at the time as well as a billabong clothes model.
I wouldn't be concerned at all because after all its living arrangements, and sometimes living with someone of the opposite gender gives you a lot to bounce off... As you said, they've been friends for ages and living together doesn't change that. The only thing living together does is put whatever relationship you have and times in by 10 and sees how you cope with it, sometimes a friend is great as a friend but as a housemate is unbearable. If she's your girlfriend, you should be placing enough trust in her to only have eyes for you, and if you're reconsidering the relationship, then question you're own motives and how much you trust and respect her because it goes both ways. Would you feel that your girlfriend would accept you if you moved in with someone of the opposite gender... there seems to be a bit of jealousy to your questions, but there shouldn't be. They're just friends sharing the same roof and a bit of independance.