My currency website supportsupports both mobile and desktop browsers at the same URLs. It isn't responsive though, it uses server side user agent detection to serve the correct version of the page.
I submit two sitemaps through webmaster tools. It appears that Google mostly ignores the mobile sitemap for me. They don't complain about it, but it doesn't report any of the pages in it are "indexed". I get mobile traffic, but the sitemap doesn't appear to be doing anything.
I leave it submitted in webmaster tools in case Google decides to do something with it in the future.