We've just began developing some mobile sites. It seems that at least one customer's mobile site appears in the desktop search results for his keyword phrase and location. In fact, the m.domain.com shows up just above his desktop URL in the Google SERP.

These are not separate sites. We have a redirect code in place to direct mobile users to the mobile version of the site. I've never seen a m.domain in a desktop search ... until now.

Is there some additional code I should be using to prevent this? Perhaps a 'disallow' in the robots.txt?