tedster - 4:50 pm on Feb 1, 2012 (gmt 0)
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.