Form Validation

[Far Cry]

I just cant seem to spot whats going wrong, its not validating the form....

<?php
require'styles/top.php';
$date = date("F d, Y g:i:s A");
?>
<br>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/
libs/jquery/1.3.0/jquery.min.js">
</script>
<script type="text/javascript" >
$(function() {
$(".submitbtn").click(function() {
var to = $("#to").val();
var subject = $("#subject").val();
var message = $("#message").val();
var dataString = 'to='+ to + '&subject=' + subject + '&message=' + message;

if(to=='' || subject=='' || message=='')
{
$('.success').fadeOut(200).hide();
$('.error').fadeOut(200).show();
}
else
{
$.ajax({
type: "POST",
url: "send.php",
data: dataString,
success: function(){
$('.success').fadeIn(200).show();
$('.error').fadeOut(200).hide();
}
});
}
return false;
});
});
</script>
<center>
<table style='font-weight:bold' cellspacing='10'><form action='send.php' method='post' name='form'>
<tr>
   <td>To</td>
   <td><select name='to'><?php $query = mysql_query("SELECT * FROM users ORDER BY username DESC") or trigger_error('Error: ' . mysql_error());
		$numrows = mysql_num_rows($query);
		if ($numrows > 0){
			while ($row = mysql_fetch_assoc($query)){
				$user = $row['username'];
                           
				echo "<option value='$user'>$user</option>"; 
			}
		}
?>
</select>
</td>
</tr>

<tr>
   <td>Subject </td>
   <td><input typ[e-'text' name='subject'></td>
</tr>

<tr>
<td>Message </td>
<td><textarea name='message' cols='20' rows='10'></textarea></td>
</tr>

<tr>
   <td><input type='submit' name='submitbtn' value='Send' class='button'></td>
</tr>
<span class="error" style="display:none"> Please Enter Valid Data</span>
<span class="success" style="display:none"> Message Sent Successfully</span>
</form>
</table>
</center>

<?php
if($_POST['submitbtn']){
$to = $_POST['to'];
$subject = $_POST['subject'];
$message = $_POST['message'];
if($to && $subject && $message){
	mysql_query("INSERT INTO messages VALUES('','$username','$to','$subject','$message','$date','0')");
	}
}
?>