function doGetCaretPosition(oField) {
    var iCaretPos = 0;
    if (document.selection) {
      oField.focus ();
      var oSel = document.selection.createRange();
      oSel.moveStart ('character', -oField.value.length);
      iCaretPos = oSel.text.length;
    } else if (oField.selectionStart || oField.selectionStart == '0') {
      iCaretPos = oField.selectionStart;
    }
    return (iCaretPos);
  }
Returns the position of the caret in the given input field
http://flightschool.acylt.com/devnotes/caret-position-woes/