It's bottle conditioned. They added some lager yeast and some fresh wort right before bottling and that's how they carbonate it. Pour the first 2/3 of the bottle in a Hefe Glass, then give the bottle a couple swirls to stir up the yeast and pour the rest in the glass. That's how you are supposed to drink a "Hefe" (Yeast) Weizen beer.
Here's one I enjoyed right before the Erdinger Tour.