如何使用$ .post函数执行函数验证值？

For example:

例如：

<script type="text/javascript" src="https://code.jquery.com/jquery-1.6.2.min.js"></script>    
<script type="text/javascript">
    $(document).ready( function() {
        function is_valid( value )
        {
           // This is what I want to do, but this doesn't works.
           return value == "Iamgood" && $.post( 'validatefield.php', 'fieldvalue=' + value, function (data) { 
                 return data == "true";
           });
        } 

        $('#formtovalidate').bind( 'submit', function() {
            if (is_valid( $('#field').val() ) return true;
            return false;
        });
    });
    </script>
<form id="formtovalidate" action="yes.php">
  <input value="Iamgood" id="field" name="field"/>
  <input type="submit" id="sendform" value="Send" />
</form>
<script type="text/javascr