You get the terrain size from TerrainData.size. The transform.position is the lower left corner (you can see this by the pivot point when a terrain is selected). Mathf.InverseLerp is good for this sort of thing:

var terrain : Terrain;
var worldPos : Vector3;

function Start () {
    var terrainLocalPos = worldPos - terrain.transform.position;
    var normalizedPos = Vector2(Mathf.InverseLerp(0.0, terrain.terrainData.size.x, terrainLocalPos.x),
                                Mathf.InverseLerp(0.0, terrain.terrainData.size.z, terrainLocalPos.z));
    var terrainNormal = terrain.terrainData.GetInterpolatedNormal(normalizedPos.x, normalizedPos.y);
    Debug.Log (terrainNormal + " " + normalizedPos.x + " " + normalizedPos.y);
}

Note that Terrain.SampleHeight uses world space and not normalized coords, so you might as well use that instead of TerrainData.GetInterpolatedHeight, since it's the same thing.