Mobile is becoming more complicated with the increasing popularity of tablets and mobile phones. As such, SEO is getting more complicated (primarily in regards to URLs). Google has tried to simplify this by getting behind responsive design and having one URL and one version of your content, but I have a hard time getting behind this as a widespread solution (more on this below).
Though this post contains guidelines for mobile SEO, keep in mind that there are always exceptions to the rule and sites should deviate from best practices.
Via Antonino Militello