Looks like the major search engines have decided on a standard way to specify a location of a sitemap in the robots.txt file:

From the ask.com blog (http://blog.ask.com/2007/04/sitemaps_autodi.html) :

Today, Ask.com, Google, Microsoft Live Search and Yahoo! together are announcing support of “autodiscovery” of Sitemaps. The new open-format autodiscovery allows webmasters to specify the location of their Sitemaps within their robots.txt file, eliminating the need to submit sitemaps to each search engine separately.

Comprehensiveness and freshness are key initiatives for every search engine, and with autodiscovery of sitemaps, everyone wins:

· Webmasters save time with the ability to universally submit their content to the search engines and benefit from reduced unnecessary traffic by the crawlers
· The search engines get information with regards to pages to index as well as metadata with clues about which pages are newly updated and which pages are identified as the most important
· Searchers benefit from improved search experience with better comprehensiveness and freshness