Several people have tried out blogging from Word 2007 (as I am now) and are having problems with .Text. This issue is in the CookComputing.XmlRpc.dll and a dateTime error they fixed. This is used in the Post struct which blows up the newPost method. If you update the dll to a later version (http://www.xml-rpc.net) then you should have no problem posting after that using the Metablog API.
For Geekswithblogs.net members, the Metablog API service URL is located at http://www.geekswithblogs.net/[YOUR USER NAME]/services/metablogapi.aspx.