Due to my casein (dairy protein) allergy, I can't have any dairy, either. But I've done SB in the past, and soy milk is a great replacement. Of course, it doesn't replace the cottage cheese, fat-free cheeses, yogurts, etc. that others get to enjoy...
I know it's doable, because I've done it. And overall, I think South Beach is a very healthy way to eat. For those of us with food allergies, it's just a bit more limiting and takes some extra planning. But then, we'll have that with ANY food plan, right?
Good luck to us both!