[rogue]

In cart_subtotal.tpl you get the smarty var value $newsubtotal and put it in a global php var $phpnewsubtotal.

{php}
  global $phpnewsubtotal;
  $phpnewsubtotal = $smarty->get_template_vars('newsubtotal');
{\php}

Then in cart.tpl you get the value from the php global var and put it in the smarty var.

{php}
  global $phpnewsubtotal;
  $this->assign("newsubtotal", $phpnewsubtotal);
{\php}

That's it I think. Be warned there is probably a more conventional way to do this as you will no doubt find out in subsequent replys by other folks.