Hi Zenchaser - I think you might be overthinking this. When I first went out, I did too. A few people simply said to me, "do you not drink?" and I just said, "No". No one cared. I had my club soda with lime/cranberry. There were a few other people who also just ordered soft drinks.
I found that I was just tired around 11 pm and I really didn't want to stay up much later, so I just went home.
I'd say if you're at a party where people are heavy drinkers, you are not going to like it anyway, because they get silly and boring the more they drink. Fortunately for me, I don't hang around those type of people now. Most are social drinkers and no one becomes a jerk.
Just have fun and plan to leave when you get tired. Relax - you're got this.