Kinda late, but you need to access the original event, not the jQuery massaged one. Also, since these are multi-touch events, other changes need to be made:

$('#box').live('touchstart', function(e) {
  var xPos = e.originalEvent.touches[0].pageX;
});

If you want other fingers, you can find them in other indices of the touches list.

UPDATE FOR NEWER JQUERY:

$(document).on('touchstart', '#box', function(e) {
  var xPos = e.originalEvent.touches[0].pageX;
});

I use this simple function for JQuery based project

    var pointerEventToXY = function(e){
      var out = {x:0, y:0};
      if(e.type == 'touchstart' || e.type == 'touchmove' || e.type == 'touchend' || e.type == 'touchcancel'){
        var touch = e.originalEvent.touches[0] || e.originalEvent.changedTouches[0];
        out.x = touch.pageX;
        out.y = touch.pageY;
      } else if (e.type == 'mousedown' || e.type == 'mouseup' || e.type == 'mousemove' || e.type == 'mouseover'|| e.type=='mouseout' || e.type=='mouseenter' || e.type=='mouseleave') {
        out.x = e.pageX;
        out.y = e.pageY;
      }
      return out;
    };

example:

$('a').on('mousedown touchstart', function(e){
  console.log(pointerEventToXY(e)); // will return obj ..kind of {x:20,y:40}
})

hope this will be usefull for you ;)

I tried some of the other answers here, but originalEvent was also undefined. Upon inspection, found a TouchList classed property (as suggested by another poster) and managed to get to pageX/Y this way:

var x = e.changedTouches[0].pageX;