Uhmm... I think this topic is acctually too old. But for anyone, who is visiting this page, because he wonders how it is done (!I'm not sure!, but I think) its done like that:
At first, without GL_DEPTH_TEST being enabled, the whole "World" is rendered as a "Wireframe". That means:
glDisable(GL_DEPTH_TEST);
glLineWidth([about] 4f);
drawModel([mode:] GL_LINES);
glEnable(GL_DEPTH_TEST);
Now, as you have rendered the "World" as a Wireframe, you just "overwrite" it with the "real" World (omg: sooo much "'s

).
Again, I'm not really sure if it is done like that, but I'm pretty sure it is. Just look at examples like blender. In blender you see an orange line around your objects in the "3D View". If you texture your object, and view it from the "back-side" (Where the Texture is getting "invisible" because it is the wrong face to show the Texture on (Culling)), you will see an orange Wireframe

.
And the second point why it may be made like that, is, that i'm pretty sure it would work like that
