XMLRPC POST for node.create not work

Anonymous users have the permission to create Article, but when I user XMLRPC POST below xml, it returns 401 Unauthorized: access denied for user anonymous , can anybody tell what's the problem? thanks in advance.

Drupal: 7.8
Service:7.x-3.0

XML content from http://drupal.org/node/1304802

<?xml version='1.0' ?>
<methodCall>
  <methodName>node.create</methodName>
  <params>
    <param>
      <value>
        <struct>
          <member>
            <name>type</name>
            <value>
              <string>article</string>
            </value>
          </member>
          <member>
            <name>body</name>
            <value>
              <string>Create body input</string>
            </value>
          </member>
          <member>
            <name>title</name>
            <value>
              <string>Test title</string>
            </value>
          </member>
        </struct>
      </value>
    </param>
  </params>
</methodCall>