Html options :onkeypress...

well, one thought would be to set a flag on the first keypress.

something like

<%= text_area ..., :onkeypress => "mykeypress()" -%>
<script type="text/javascript">
var key_flag = false;
function mykeypress() {
  if(key_flag == false) {
    key_flag = true;
    new AjaxRequest(...);
  }
}
</script>