Our site currently has the sitemap index located in the robots.txt, it is also added to the webmaster tools. It's been about 6 months and not many pages have been picked up. (http://www.preamped.com/PA_SiteMapIndex.XML.gz)
Does the crawler support compressed site map files? Is that the issue? Do sitemaps really need to be named "Sitemap.XML" even if the changed name is in the robots.txt?
Having 450 indexed pages with a sitemap of over 2 million is pretty sad after 6 months. We just don't know what to try next.