That's absolutely right, only triangles can guarantee that they'll always lay on a plane regardless of what you do with the vertices; any other polygon is easily distorted off-plane thus producing results depending on how it is calculated. In my next test I'ill try to raise two opposite vertices of a square - but I strongly doubt I'll get the saddle-shaped hyperboloid one would await.
There might be a law against using AC3D in the office, its a real drug! Good enough that there's still the excellent excuse "I wanted to see how it looked like before having it machined"
P.S.: congratulation for supercoldmilk.com, it's a fine a site if I ever saw one!