In a recent iSEO panel at SMX Advanced, Pierre Far, Google Webmaster Trends Analyst, has announced its official recommendations and clear guidelines regarding mobile SEO. According to Overview of Google's Recommendations for Optimizing Smartphones found here - “Google recommends webmasters to follow the industry best practice of using responsive web design, namely serving the same HTML for all devices and using only CSS media queries to decide the rendering on each device.†What is Responsive Design Approach? It is where a web site will use Cascading Style Sheets 3 (CSS3) media queries in altering the way the page renders on mobile devices. In this case, websites will now have one URL, one content, one HTML code but CSS media queries to specify which CSS rules apply for the browser displaying the page. Why Use Responsive Design Approach for Mobile SEO? • Google says the advantages of this method include having one URL which will make it easier for your users to interact with the page, share and link to. • Having one URL and same HTML is also more efficient when Google is crawling your content as Google doesn’t have to crawl multiple pages; thus, Google can better crawl more of your site. However, for those who are going with the dynamic serving configuration, you are able to do so. Google says they also do support the dynamic serving method with either the same URL or a different URL. But, Google recommends that if you go those routes, you are advised to use the Vary HTTP header as a hint for GoogleBot-Smartphone to crawl the site.
I'd agree with that but there are some content issues which still require serverside decisions - such as serve up flash or go with basic html?
i did a bit of a write up on my blog about mobile seo and the different options of using multiple urls or css to serve up content to the mobile browser