Mouse Camera, Camera Mouse

Right. I have tried asking many many people how to do this script and no one seems to get what it is I mean. Basically I want this to be a valid script

function Update () {        
transform.position = Vector3(Input.mousePosition);
}

Which frustratingly it isn't. I want the mainCamera x and y co-ordinates to equal the x and y coordinates of the mouse cursor. It should be simple but it has given me a lot of difficulty.

I do have a script that almost works but I don't think I'm going in the right direction so I'm not going to put it up because it just confuses people usually.

Input.mousePosition is a Vector3 already....

Are you working in C# or javascript? You tagged the question with c#

in C# (C# scripts must be named the same as their class name, in this example CamMouse)

using UnityEngine;
using System.Collections;

public class CamMouse : MonoBehaviour {

    void Update () {
        transform.position = Input.mousePosition;
    }
}

in Javascript

function Update () {
	transform.position = Input.mousePosition;
}

or, for fun and syntax similar to yours, in C#

using UnityEngine;
using System.Collections;

public class CamMouse : MonoBehaviour {

    void Update () {
        transform.position = new Vector3(Input.mousePosition.x,
             Input.mousePosition.y,
             Input.mousePosition.z);
    }
}

Also, note that Input.mousePosition goes between (0,0,0) in the lower left corner, to (Screen.Width, Screen.Height, 0) in the upper right.