float4 WorldPos = ...; //I assume you have this?
float4 colour = tex2D(MyTexture, WorldPos.xy); //Uses x and y components as texture coordinates.

float4 colour = tex2D(MyTexture, WorldPos.xy / 100.0f); //Uses x and y comonents as texture coordinates.

fragment_program MyFragmentShader1 cg
{
   source WorldCraftTerrainShaders.cg
   entry_point MyFragmentShader1
   profiles ps_1_1 arbfp1
   default_params
   {
      param_named_auto world world_matrix
   }

}

vertex_program MyVertexShader1 cg
{
   source WorldCraftTerrainShaders.cg
   entry_point MyVertexShader1
   profiles vs_1_1 arbvp1

   default_params
   {
      param_named_auto worldViewMatrix worldviewproj_matrix
      param_named_auto world world_matrix
   }
}

material Worldcraft/Greengrass
{
   technique
   {
      pass
      {
         fragment_program_ref MyFragmentShader1
         {
         }
         texture_unit
         {
            texture grass_1024.jpg
         }
      }
   }
}

void MyFragmentShader1(float4 position : POSITION,
                       float4 worldPosition : TEXCOORD0,
                       out float4 color: COLOR
                       uniform float4x4 world)
{
   worldPosition = mul(world, position);
   color = tex2D(texture, WorldPosition.xy / 10.0f);
}

void MyVertexShader1(float4 position : POSITION,
                     out float4 oPosition : POSITION,
                     out float4 worldPosition : TEXCOORD0,
                     uniform float4x4 worldViewMatrix,
                     uniform float4x4 world)
{
   

}