CODE:

        D3DXVECTOR3 v;
        v.x = ( ( ( 2.0f * ptCursor.x ) / pd3dsdBackBuffer->Width ) - 1 ) / pmatProj->_11;
        v.y = -( ( ( 2.0f * ptCursor.y ) / pd3dsdBackBuffer->Height ) - 1 ) / pmatProj->_22;
        v.z = 1.0f;

        // Get the inverse view matrix
        const D3DXMATRIX matView = *g_Camera.GetViewMatrix();
        const D3DXMATRIX matWorld = *g_Camera.GetWorldMatrix();
        D3DXMATRIX mWorldView = matWorld * matView;
        D3DXMATRIX m;
        D3DXMatrixInverse( &m, NULL, &mWorldView );

        // Transform the screen space Pick ray into 3D space
        vPickRayDir.x = v.x * m._11 + v.y * m._21 + v.z * m._31;
        vPickRayDir.y = v.x * m._12 + v.y * m._22 + v.z * m._32;
        vPickRayDir.z = v.x * m._13 + v.y * m._23 + v.z * m._33;
        vPickRayOrig.x = m._41;
        vPickRayOrig.y = m._42;
        vPickRayOrig.z = m._43;