Sitemaps are great tool to make search engines better crawl and index your site, and since Google, Yahoo and Microsoft joined together to adopt this protocol you will be more interested now to have a better way to tell them about its location. The answer is in the Sitemaps protocol, the first alternative via robots.txt you indicate :
Sitemap: <sitemap_location>
And the second alternative using HTTP request
<searchengine_URL>/ping?sitemap=sitemap_url
You have to replace <searchengine_URL> with the URL provided by the search engine. Today there is only three big search engines adopting the protocol, but it will be more useful if there will be an automated tool for publishers to ping automatically every time their Sitemaps changes.
