Its flat right now in game. However he was talking about when cataclysm is released flying from one continent to the next. Most of Azeroth (with the exception of Northrend) isn't really there. That is why it is taking them so long to make cataclysm because its just 3D art that if you could fly right now you would be able to fly right through. Take Org or SW for example you can go underneath it. There isn't ground there just a giant hole. Most of the mountains are like that too.
It would be plausible to fly from one to the next in Cata however dont expect it