http://learn.javascript.ru/play/HiS1Fb
<!DOCTYPE HTML>
<html>
<head></head>
<body>
<textarea id="message"></textarea>
<script>
var message = document.getElementById('message');
message.onpropertychange = message.oninput = function () {
this.value = this.value.toLowerCase();
};
</script>
</body>
</html>