ciao AleX, la sintassi non sarà elegantissima ma sembra funzionare:

codice:
<script type="text/javascript">

function formatIt(s) {
var oldfirst="";
var second="";
var newfirst="";
var y=0;
var result="";
if (s.indexOf('.')>-1) {
 oldfirst=s.substring(0, s.indexOf('.'));
 second=s.substr(s.indexOf('.')+1, 2);
 for (var i=oldfirst.length-1; i>=0; i--) {
  if (y%3==0 && y>0) newfirst='.'+newfirst;
  newfirst=oldfirst[i]+newfirst;
  y++;
 }
result=newfirst+','+second;
}
else {
 for (var i=s.length-1; i>=0; i--) {
  if (y%3==0 && y>0) newfirst='.'+newfirst;
  newfirst=s[i]+newfirst;
  y++;
 }
result=newfirst;
}
return result;
}

function divisione(ff) {
  var A = parseFloat(ff.CAMPO1.value);
  var B = parseFloat(ff.CAMPO2.value);
  ff.CAMPODEST.value = formatIt((A/B).toString());
}
</script>
fammi sapere. :)