This was picked up from a thread in Google Webmaster Help.

Google: You Should Not Have A Mobile Sitemap

If you deploy any of the three mobile configuration types and they are deployed appropriately, you do not need a mobile XML sitemap file.
In fact, Google's John Mueller said you should NOT have a mobile sitemap for mobile sites. The only case you should have a separate mobile site is when you have a feature-phone (WAP/WML, etc) version of your site, which is something most people don't bother with these days.