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You can do either option for the sitemap file.  Kate Morris has a nice post around using multiple sitemaps here: http://moz.com/blog/multiple-xml-sitemaps-increased-indexation-and-traffic but it's mostly a question of site size.  Congrats on getting your mobile site up and running!

Agreed! Thank you for that Massimiliano. Definitely a lack of deeper browsing is of concern.

Hi Erin, thank you for your response. Ok that makes sense. I appreciate it!