RaycastHit.barycentricCoordinate  

// Attach this script to a camera and it will
// draw a debug line pointing outward from the normal
function Update () {
// Only if we hit something, do we continue
var hit : RaycastHit;
if (!Physics.Raycast (Camera.main.ScreenPointToRay(Input.mousePosition), hit))
return;

// Just in case, also make sure the collider also has a renderer
// material and texture
var meshCollider = hit.collider as MeshCollider;
if (meshCollider == null || meshCollider.sharedMesh == null)
return;

var mesh : Mesh = meshCollider.sharedMesh;
var normals = mesh.normals;
var triangles = mesh.triangles;

// Extract local space normals of the triangle we hit
var n0 = normals[triangles[hit.triangleIndex * 3 + 0]];
var n1 = normals[triangles[hit.triangleIndex * 3 + 1]];
var n2 = normals[triangles[hit.triangleIndex * 3 + 2]];

// interpolate using the barycentric coordinate of the hitpoint
var baryCenter = hit.barycentricCoordinate;

// Use barycentric coordinate to interpolate normal
var interpolatedNormal = n0 * baryCenter.x + n1 * baryCenter.y + n2 * baryCenter.z;
// normalize the interpolated normal
interpolatedNormal = interpolatedNormal.normalized;

// Transform local space normals to world space
var hitTransform : Transform = hit.collider.transform;
interpolatedNormal = hitTransform.TransformDirection(interpolatedNormal);

// Display with Debug.DrawLine
Debug.DrawRay(hit.point, interpolatedNormal);
}