It looks like Google and Yahoo! have gotten together to encourage an open standard for sitemap descriptor files for use by Webmasters to notify Search Engines of changes to their website.
A video interview on WebProNews.com explains in detail:
The site: sitemaps.org, explains some of the frequently asked questions as well as a detailed description of the format.
There are tools already that allow users to create these sitemaps for Googles Sitemap system, unless there are any major changes to the open format it would be safe to assume these tools will work for future submissions.
If you have a new site, or a site that you update frequently with dynamic urls, these sitemaps can be a great tool to tell the Search Engines exactly which pages they should be looking at.
All in all, its nothing too big, but certainly a great step forwards in regards to co-operation between the Search Engine giants.