jquery kullanmadan ise:
<html>
<head>
<script type="text/javascript">
function disable()
{
if(document.form.textarea.value=="")
{
document.form.submit.disabled=true;
}
else
{
document.form.submit.disabled=false;
}
}
</script>
</head>
<body onload="javascript:disable();">
<form name="form">
<textarea name="textarea" rows="2" cols="10" onkeypress="javascript:disable();"></textarea>
<input type="submit" name="submit">
</form>
</body>
</html>