The Bing404 plugin for WordPress is a new plugin that returns a page of intelligent search results in place of the standard WordPress 404 error page by using keywords extracted from the dead URL that otherwise would have resulted in a ‘Page Not Found’ error.
This plugin should work out-of-the-box with most themes using the included 404 template. You can also define your own to fine tune the look and feel to match your theme.
The plugin is part of the Bing Search Library for PHP project and was created in conjunction with PHP guru Cal Evans, who has posted a tutorial and details about the plugin on php | architect at: http://www.phparch.com/2010/06/04/wordpress-404-plugin-built-on-bing-wrapper/
More at: http://www.bing.com/community/blogs/developer/archive/2010/06/03/bing404-plugin-for-wordpress.aspx