Trouble switching cameras

I'm trying to create a script that switches between cameras but when I run this code nothing happens. BTW I'm not getting any errors either - simply nothing happens.

Can anyone spot any flaws with what I've done so far.

var camera1 : Camera;
var camera2 : Camera;

function Start () {    
camera1.enabled = false;    
camera2.enabled = true; 
} 

function OnGUI () {
    if (GUI.Button (Rect (10,10,150,100), "Play Animation")) {
        camera1.enabled = false; 
        camera2.enabled = true; 
        animation.Play ("Camflyby");
    }
}

Enable camera1 and disable camera2 in your GUI button, not the other way around.