I think it depends where you live, because where I am they do not consider you positive or offer abx just because you tested positive in a previous pregnancy. They test you at each pregnancy and go by those results. The only time the status "carries" over is if you had a GBS infected baby in a previous pregnancy (not the same as mama being colonized). Of course, this protocol depends on where you live, so what is true around here might not be true for you. However, if you test negative in this pregnancy after testing positive in the same pregnancy, that's a different story.
About being born in the caul: Although it is semi-permeable, so long as the membranes don't rupture it's very unlikely that your baby will come in contact with the GBS bacteria, so babies born in the caul are pretty much protected. I'm actually praying for an "in-caul" birth for that reason, though it's usually 1 in 80,000 so certainly not worth it to hold my breath ;-) Also, in terms of abx, you need them to be in your system for 4 hours for them to work, so it's not helpful to wait for the water to break to take the abx because if that happens within 4 hours of the baby being born, the abx that you took would essentially be useless.
However, I agree with Mama Harrison about hibiclens; a lot of midwives hold by it and even say that it is less evil than abx because it doesn't cross the membranes, but there really isn't enough information about pre-natal use, only intrapartum use - and in terms of intrapartum use, it has been shown to be as effective as abx, but not necessarily safe for you or the baby. It's still chemicals and you are still douching, and to me both of those things sound counter-intuitive in pregnancy.
I just have to say that I could talk for hours about this issue, as could many people who have encountered it personally, and the reason is because it is so full of grey areas, from testing to diagnosis to treatment. Everyone has to choose the regimen they feel best about. For whatever reason, perhaps because I've got close ties to some horror stories about it or perhaps because of my own fears as a first-time mother, especially giving birth at home, GBS scares me and I feel the need to take care of it. Other people, less exposed to those horror stories and perhaps with different fears in pregnancy, might look at those same risk factors and decide that it is not necessary to treat.
I decided to test and came out positive. I did the chlorhexidine treatment, VERY reluctantly, for a week before deciding I just couldn't do it anymore. Since I first tested, I've been taking 5 billion units of acidopholus a day plus 1200 mg of garlic, and 1000 mg of vitamin C, in addition to prental vitamins and omega 3. I retested last night and I'll see what happens in a couple of days; if I come out negative, I will probably only take the abx if I encounter the risk factors (PROM or high fever). If I retest positive, though, I plan on taking abx from active labor and on. I've found a midwife who will do it at home, and decided this is the best of both worlds - yes I'll be taking abx, but at least neither me nor baby will be exposed to hospital germs. This is my decision, however, and I know others would feel differently. And regarding probiotics, you can give it to the baby after it is born; just ask your midwife for exactly how much and how. It's not a perfect fix and obviously a drug free birth would be best, but it really adds up to what makes you feel most comfortable.