Hi Julian,

Here's an article, "The Tweet Effect",  I read awhile ago on the SEOMoz blog that sounds like it will help support you. It was an experiment on how social media (in this case Twitter) can impact rankings. Not necessarily a quote, but I think you will be pleased to read the results and could find a way to factor it into your presentation.

Google and other search engines know about XML files.  The reason I don't necessarily say indexed is because you don't see them really popping up on a SERP page.  Robots know the difference between a straight xml file and an rss feed because of the technical markup and structure of the actual file.

For example a very common use for Google and xml files is sitemaps, which you can give them the sitemap feed in Webmaster tools so they will know about the file.  If you did a search for www.rootdomain.com + sitemap file you would never see an .xml file actually popup.  You would need to visit the url directly.