Not sure Matt is the expert on mobile search indexing at Google, as he largely referenced the Japanese blog post, and the Japanese results seem to be more sophisticated than the US results, as they employ blended mobile ranking algorithms to prioritize mobile content. Nonetheless, if you haven’t seen Matt’s latest YouTube video, “Does indexing a mobile website create a duplicate content issues [sic]?“, be sure to check it out. He basically says there should be no duplicate content issues if you serve your mobile content to Googlebot Mobile and desktop content to Googlebot, which is also in Google’s new SEO Starter Guide. What’s interesting about the video is that Matt recommends m.*.com for usability purposes, and I’ve never seen anyone at the engines officially recommend creating mobile-specific URLs before. As I mentioned before, it is the most popular mobile URL option, but I know that some people are hesitant to create mobile URLs for fear of splitting link popularity.
Definitely worth a look. I doubt this will be the last word from the engines on mobile SEO, as for me there are a lot more questions that come out of this than answers. Nonetheless, it’s great to see Matt Cutts recognize that people are building mobile sites, and starting to tackle some of the SEO issues that arise from that.