WordPress.com automatically provides sitemaps for all blogs they host.
XML Sitemaps for Search Engines
Your blog is enabled with just about everything you need to be super-searchable. This includes an XML sitemap, which you can access at yourblogname.wordpress.com/sitemap.xml.
No need to do anything extra on your end. The sitemap file included with your blog is available to every search engine that supports the protocol, including Google, Yahoo!, Bing, Ask.com, and others. WordPress.com automatically sends updates to search engines every time you update or delete a page or post. If you would like to learn more about the protocol, visit sitemaps.org. http://en.support.wordpress.com/sitemaps/