You can have one cup....but 1/2 cup is a serving...meaning you can have two servings a day (mind you, they all count towards your two cups daily dairy).
Edit: Sorry, you can have two cups per day if you want it, but 1/2 cup is still a serving.
I thought it was two cups of dairy a day "including yogurt." I had always taken that to mean you could have two cups if you wanted it, but no other dairy. i'd be interested in a solid answer to this as well.
"MILK/DAIRY
Except for half and half, 2 cups allowed daily, including nonfat yogurt or lowfat plain yogurt"
So if you want to make it all yogurt, it's OK. However, be aware that non-fat yogurt has 137 calories per cup and non-fat milk only 86.