I have been thinking about this also. The risk is that when the cake thaws, it will settle on the icing and create bulges. My guess is that if the cake is a firm one, like madeira, it wouldn't bend much when thawed, but it a softer cake it might. Anyone?
I haven't done these, just admired and wondered like a lot of others. But on another site (egullet) I read a post from someone who claims to have it on authority that cakework use edible ink printers to make these.