Real simple actually. Just have the position of the GUIText be based on the camera viewport.

var viewportPosition : Vector3;  //Vector3(0,0,0) I think is bottom left, don't worry about the z position

function Update ()
{
    transform.position = Camera.main.ViewportToWorldPoint(viewportPosition);
}

Hope this helps.

EDIT: To find the correct viewport position. Add this to Update function and move to desired position and the viewport position will print in console.

print(Camera.main.WorldToViewportPoint(transform.position));