The latest release of WordPress 2.9.1 has a minor bug in the xmlrpc.php file which prevents the user from creating a new post remotely. There are quite a lot of issues with the xmlrpc.php, one being that the xml returned is not well-formed and the other is calling an method on an array.
I ran across these issues when I was trying to import some posts into WordPress using XMLRPC with Zend_XmlRpc_Client.

Anyway, the patch here will allow you to insert the post:

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Index: xmlrpc.php
===================================================================
@@ -2195,13 +2195,16 @@
 
        // Do some timestamp voodoo
        if ( !empty( $content_struct['date_created_gmt'] ) )
-           $dateCreated = str_replace( 'Z', '', $content_struct['date_created_gmt']->getIso() ) . 'Z'; // We know this is supposed to be GMT, so we're going to slap that Z on there by force
+           $dateCreated = $content_struct['date_created_gmt'];
        elseif ( !empty( $content_struct['dateCreated']) )
-           $dateCreated = $content_struct['dateCreated']->getIso();
+           $dateCreated = $content_struct['dateCreated'];
 
+
        if ( !empty( $dateCreated ) ) {
-           $post_date = get_date_from_gmt(iso8601_to_datetime($dateCreated));
-           $post_date_gmt = iso8601_to_datetime($dateCreated, GMT);
+           $dateIXR = new IXR_Date(strtotime($dateCreated));
+           $dateCreated = $dateIXR->getIso();
+           $post_date     = get_date_from_gmt(iso8601_to_datetime($dateCreated));
+           $post_date_gmt = iso8601_to_datetime($dateCreated. "Z", GMT);
        } else {
            $post_date = current_time('mysql');
            $post_date_gmt = current_time('mysql', 1);

Copy the above code to a file, call it xmlrpc.patch. To patch the content of xmlrpc.php which is located say under directory public:

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~/public$ patch -p0 < xmlrpc.patch