Basing a grid off the terrain in unity.

Yes, check all the terrain script on the Unity Wiki, all by Eric5h5, like I have found while using these for research myself. He Is .... Everywhere! (in a good way, I would be lost without his advice as found on all his answers and freely provided code. Thanks again Eric, you are a Legend). You would be wise to read his terrain scripts and learn from them : http://wiki.unity3d.com/index.php/Scripts/Editor

For more stuff you are looking at, watch quill18creates on YouTube : http://www.youtube.com/watch?v=dDqCPMpX1vI

Edit : Here's how to read the heights of the terrainData. Using this script as a reference : http://wiki.unity3d.com/index.php/RaiseHeightmap

 #pragma strict
 
 var myTerrain : Terrain;
 var myTerrainData : TerrainData;
 
 function Start() 
 {
     if ( !myTerrain )
     {
         myTerrain = Terrain.activeTerrain; // find the active terrain
     }
     
     myTerrainData = myTerrain.terrainData; // store the terrainData
     
     ReadTerrainAndPlaceCube();
 }
 
 function ReadTerrainAndPlaceCube() 
 {
     var heights : float[,] = myTerrainData.GetHeights( 0, 0, myTerrainData.heightmapWidth, myTerrainData.heightmapHeight );
     
     for ( var y = 0; y < myTerrainData.heightmapHeight; y ++ ) 
     {
         for ( var x = 0; x < myTerrainData.heightmapWidth; x ++ ) 
         {
             if ( x % 20 == 0 && y % 20 == 0 ) // place a cube every 20*20 units
             {
                 var actualHeight : float = heights[y,x] * myTerrainData.size.y; // need to multiply the height by the size.y of the terrain
                 
                 var cube : GameObject = GameObject.CreatePrimitive( PrimitiveType.Cube ); // create a cube
                 
                 cube.transform.position = new Vector3( x, actualHeight, y ); // position it at the actual height
             }
         }
     }
 }