Blender internal render offers a method called "window"-mapping, which forces the texture to ignore perspective influences and align relatively to the camera/window, like it is flat or static. Unfortunately the Blender game engine does not support this method - so I tried to recreate this effect. It is a very easy node-setup which can be, of course, mixed with other materials.

http://www.youtube.com/watch?v=zCWPjLF4jes