Why cant i subdivide? I have looked every were and cant find out why I cant subdivide this face. I split a default sphere and filled the top with a face and it wont let me subdivide that face. I need to so i can  use the displace modifier on it!
In fact, it does subdivide but you don't see it because the scene is in edge selection mode. And as there is no vertex in the center, only border edges are divided.

But what you can use is a grid fill CtrlF then "grid fill":