tedster - 8:17 pm on Aug 13, 2009 (gmt 0) Here's a related thread on Google's forums, with a reply from Google's John Mueller (JohnMu) [google.com...]
Are you seeing the sitemap actually rank for keywords? Or is it just showing up in your site: query results? The main thing I'd suspect is that somewhere there is a link pointing to this sitemap file.
It looks like the URL of your Sitemap file is included in another one of your Sitemap files -- which is likely the reason we crawled and indexed it :). If you don't want them indexed, using the x-robots-tag HTTP header element is a great idea. An alternative would be to use a different Sitemap URL and remove this one with the URL removal tool.
Here's a related thread on Google's forums, with a reply from Google's John Mueller (JohnMu)