I can't find any specific instructions about a sitemaps index, either. I always assumed that the format was the same no matter what kind of sitemaps were being linked to - regular, mobile, video, whatever. I have seen that in practice, as well.
You might check the robots.txt file for a couple sites that you know are major and have a dedicated mobile site - just to see what the major players do.
Just an FYI - if your mobile version is on the same domain as your regular version, you may get warnings about duplicate titles or descriptions in WMT. However, I have not seen any warnings about duplicate content. Supposedly, using the mobile site map lets Google know it is not to be treated as duplicate content.
Thanks, Marshall. That page does offer the XML format for a mobile sitemap. But I still don't see any instruction for a mobile Index file when there is more than on sitemap involved - such as happens on bigger sites when a single sitemap goes beyond 50,000 URLs (10MB unzipped.)
I've been assuming that the format is the same for specialized sitemaps as it is for the general multple sitemap case - as given here: [support.google.com...]
[edited by: tedster at 6:56 pm (utc) on Feb 1, 2012]
Google can accept Web Sitemaps in a number of formats, but we recommend creating a Sitemap based on the Sitemap protocol because the same file can be submitted to the other search engines, such as Bing and Yahoo!, that are members of sitemaps.org.