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>