Bit of fun experimenting on this one. That blur was achieved using a hand-painted depth map, which is a concept I 'borrowed' from 3D. A depth map is a grayscale image that represents the depth information in a still picture. Black represents short distance, white represents far distance (and everything in between). After I painted the depth map on a separate layer, I fed it into Photoshop's Lens Blur filter and dialed in the amount of blur I wanted.