At SMX in June, Googler Pierre Far, discussed responsive design, and highlighted an important change in Google's stance on Mobile website optimisation - both for smart & feature phones.
This was followed up by Far announcing an updated Google Developer page on building smartphone optimised websites, which highlighted their preference for responsive web design solutions.
This is a major change, and also carries a number of benefits for site owners - particularly big brand websites currently also hosting App services.
Previously, Google's advice had been to build out a separate URL structure for mobile devices, supported with a mobile XML sitemap to identify the URLs intended for Google's mobile bot & index.
Unfortunately, in the real world, SEOs would repeatedly see mobile pages indexed in Google's main index, undistinguished from the desktop-intended URLs, except for their terrible bounce rates as visitors landed on pages designed for mobile devices on their 27" desktop screens.
At QueryClick, we've been advocating a responsive design solution using a single core set of URLs to serve all target devices for some time now, partly to resolve this poor implementation by Google, but also to reduce our clients' technical outlay and legacy costs in supporting supplementary apps to deliver their website services / functionality on smartphone devices: app codebases are fundamentally different from plain HTML5, and so require additional staff or external support to execute and upkeep.
By delivering a site that uses HTML5 and, in particular, CSS3 media queries to deliver a designed experience for desktop, mobile, smartphone, or tablet devices, extremely efficient deployments can be used which have the added benefit of consolidating domain value to the minimum required URLs, boosting overall SEO performance - in many cases quite significantly.
With Google's statement of support for this approach, it's safe to say that any company undertaking a site redevelopment should strongly consider implementing HTML5 & CSS3 to deliver their site across all target devices for the benefit of their SEO performance and reduced operating costs.