You can activate and deactivate cameras if you can get a hold of the object that holds them.

//cameraObject is the gameObject which holds the camera

cameraObject.camera.active = false;

So If you had a script which kept track of all the cameras in your scene then you could switch cameras easily. For example here is a function which would go through an array of camera objects and turn a certain one on (based on the index sent to it).

var cameras : GameObject[]; 

function SelectCamera (index : int) {
for (var i : int=0 ;i<wcameras.length; i++) {
    // Activate the selected camera
    if (i == index){
        cameras[i].camera.active = true;
    // Deactivate all other cameras
    }else{
        cameras[i].camera.active = false;
    }

}

You could use this function by calling it with a message

BroadcastMessage ("SelectCamera", 1);

or by outright calling it

SelectCamera(1);

You just need to get hold of the instance of the script it's in

DastardlyBanana's answer would be more flexible, but i'll post my (very simple!) answer here anyway:

You could check for a collision with a collider and then enabled/disable your cameras, so:

var Camera1 : Camera;
var Camera2 : Camera;

function Start(){
    Camera1.enabled = true;
    Camera2.enabled = false;

}

function OnTriggerEnter (other : Collider) {

    if(other.gameObject.name == "switch_camera"){

        Camera1.enabled = false;
        Camera2.enabled = true;

    }
}

function OnTriggerExit (other : Collider) {

    if(other.gameObject.name == "switch_camera"){

        Camera1.enabled = true;
        Camera2.enabled = false;

    }
}

Camera1 and Camera2 are assigned in the editor, or you could assign them with code.

Cheers.

Thanks because I've just looked at your answer I had a lot o thngs to do but it's not as interresting as your answer thanks to you 2!!