The new tool works differently then other sitemaps tools. The tool "finds new and modified URLs based on your webserver's traffic, its log files, or the files found on the server," said Google. Plus the tool will send pings to Google Blog Search for all new or modified URLs in the XML file.
It is pretty neat to use site traffic as a method of determining which pages to include in your Sitemap file.
Google's JohnMu said in a Google Webmaster Help thread that it runs on both Microsoft IIS and Linux Apache servers and:
While it's not for everyone (you need to have administrative access to the server for one), I think it's pretty cool :-).
Forum discusison at Google Webmaster Help.