From Bryson Meunier at SEL: "Knowing that search engines index and return mobile content for relevant queries, and that they have blended mobile ranking algorithms to rank mobile content for mobile queries and desktop content for desktop queries, it’s clear to me that they don’t treat mobile sites like traditional desktop duplicate content. Knowing this, it’s a wonder that many mobile SEO experts, including some whose opinions I generally respect, would continue to recommend blocking your mobile site from Googlebot and other web crawlers."