po, eu to testando aqui e tá funcionando cara copia o código completo e testa denovo <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
<script language="JavaScript">
function retira_acentos(palavra) {
com_acento = 'áàãâäéèêëíìîïóòõôöúùûüçÁÀÃÂÄÉÈÊËÍÌÎÏÓÒÕÖÔÚÙÛÜÇ´`';
sem_acento = 'aaaaaeeeeiiiiooooouuuucAAAAAEEEEIIIIOOOOOUUUUC ';
nova='';
for(i=0;i<palavra.length;i++) {
if (com_acento.search(palavra.substr(i,1))>=0) {
nova+=sem_acento.substr(com_acento.search(palavra.substr(i,1)),1);
}
else {
nova+=palavra.substr(i,1);
}
}
return nova;
}
</script>
</head>
<body>
<form >
<textarea onkeyup="this.value=retira_acentos(this.value);"></textarea>
</form>
</body>
</html>