This is an excellent new step in the world of SEO. Google, MSN and Yahoo! have teamed up to provide a uniform protocol for submission of sitemaps to all three search engines.
The above line can be placed anywhere within the robotx.txt file, which is widely accepted as the standard robots access / exclusion protocol by all search engines and is typically the first file accessed by the search engine spider when it accesses any site.
The Live Search blog provides a detailed post at
Sitemaps auto-discovery by Google, Yahoo! and MSN Live Search