If you are implementing two separate versions of site like site.com for desktop users and m.site.com for mobile users then it is necessary to use rel=canonical tag for your m.site.com. This will provide the full credit to the desktop version of the site and help it to rank better. The Page Rank would not get divided between the desktop and mobile versions.
But, having a responsive design allows to have a single version of the site serving both the desktop and the mobile users.
Here is a video from Matt Cutts explaining the confusion between SEO value and responsive design.
Also See:-
How to Design SEO Friendly Web Page?
Navigation in Seo
Orphan Page
Breadcrumbs
Site Wide Links
Doorway Pages
RSS feeds
Rel Canonical Element
Image Optimization
Benefits of SEO
