You can use Camera.WorldToViewportPoint: it returns the object position in viewport coordinates, which the GUITexture understands (GUITexture script):

var target: Transform; // drag here the object to follow

function Update(){
  // place the GUITexture at the target's position:
  transform.position = Camera.main.WorldToViewportPoint(target.position);
}

NOTE: Usually you want to draw the texture at some fixed offset of the target, so that it doesn't occlude the object. For GUITextures, usually it's better to define a world offset, and adjust the screen position in the Inspector, modifying the Pixel Inset fields:

var target: Transform; // drag here the object to follow
var offset: float; // world height above the target position

function Update(){
  // calculate the position including the vertical offset:
  var pos = target.position + offset * Vector3.up;
  transform.position = Camera.main.WorldToViewportPoint(pos);
}