According to Google Blog, Yahoo and MSN joined in to officially support Sitemap. While using sitemap does not guarantee that your website will be automatically included in the search engines, it does provide “helpful hints” to search engine crawlers to do a better job crawling your website.
According to the web statistics of my websites, the sitemaps that were generated specifically for Google Sitemaps, were often accessed by both MSN and Yahoo boots for quiet some time now.
As a WordPress user you may benefit from using Google Sitemap plugin for your site.