Something

<form action="autoreply.php" method="POST" name="autoreply">
		<textarea name="subject" rows="3" cols="25" onblur="if (isblank(this.value)) {this.value=document.getElementById('autoreply').elements['subject'].value;;}">default_subject</textarea>					<textarea name="body" rows="9" cols="45" onblur="if (isblank(this.value)) {this.value=document.getElementById('autoreply').elements['body'].value;}">... default_body</textarea>
		<input type="submit" name="new_autoreply" value="enable an autoreply">
</form>