Try this:

<SCRIPT LANGUAGE=JAVASCRIPT>
function getCursorPos(textElement){
 var cursorPos = -1;
 if (textElement && textElement.createTextRange) {
  var range = document.selection.createRange().duplicate();
  range.setEndPoint('StartToEnd',range);
  var start = document.body.createTextRange();
  start.moveToElementText(textElement);
  cursorPos = calcBookmark(range.getBookmark())-calcBookmark(start.getBookmark());
  var rLen = 0;
  do{
   var BrLen = rLen;
   rLen = newLine(textElement.value.substring(0,cursorPos + rLen + 1));
  }while(BrLen!=rLen);
  cursorPos += rLen;
 }
 return cursorPos;
}
function calcBookmark(bk){
 return (bk.charCodeAt(0)-1)+(bk.charCodeAt(3)-1)*65536+(bk.charCodeAt(2)-1);
}
</SCRIPT>
<SCRIPT LANGUAGE=VBSCRIPT>
option explicit
function newLine(str)
 dim nl, r
 set nl = new RegExp
 nl.global = true
 nl.pattern = "\r\n"
 set r = nl.Execute(str)
  newLine = r.count
 set r = nothing
 set nl = nothing
end function
</SCRIPT>
If you rather like to use pure javascript, replace the newLine function with a split('\n');