Bryson Meunier wrote a piece at Search Engine Land named Don't Penalize Yourself: Mobile Sites Are Not Duplicate Content. The article basically explains that if you have a mobile web address, i.e. different URLs for your mobile content, with the same content you have on your main URLs, it is NOT duplicate content. He said, Google is smart enough to figure out your mobile content is not duplicate to your main content - because they are smart enough to know when to show you the mobile content versus the normal content.
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.
A Sphinn thread has a comment about this from Sebastian who said:
So many webmasters and SEOs are totally ignorant WRT more or less identical content presented on different platforms vs. duplicate content flooding a single channel. Hopefully this piece pushed to the front page will enlighten many of them.
Honestly, what I still don't get is why are webmasters still creating different URLs for mobile content? Why aren't they detecting useragents and showing a different stylesheet for those mobile handsets? I really don't understand why bother with special URLs? I mean, yea, Google is smart enough to know a mobile URL from a non-mobile URL based on the content. But when someone links to your mobile URL, that link doesn't help you as much as it linking to your main URL. I rant about this more over here and give pretty good examples of how this works.
Forum discussion at Sphinn.