I've been building a lot of ASP.NET MVC and Orchard CMS websites using both responsive web design and jQuery Mobile, so anything that catches my eyes on this subject is fair game for awhile :) In the past, Google hasn't been too clear on the subject of improving and optimizing Google SEO when targeting smartphones, tablets, and desktops. Recently, however, Google came out with a clear strategy and set of recommendations on how to best maximize SEO when developing websites, and the clear winner is Responsive Web Design.
Unless you need a completely different user experience, the simplest way to target multiple devices using your website is to use media queries that adapt to the screen size of the device and ideally implement some server-side techniques to help with responsive images for smaller devices ( iPhone and Android phones ) until the "srcset" solution for images gets approved and implemented by browser vendors.
Google prefers you use responsive web design to maximize SEO, because it follows the one URL, one Content policy making it easy to handle page ranking, website crawling, and website indexing. Delivering different website content using the same URL based on device doesn't follow the one URL, one Content scenario and makes it troublesome for Google to crawl a website, rank its content, and index it. Obviously Google has to crawl the website with different user agent headers and as a website developer you need to annotate your website to give Google a heads up on your mobile development strategy.
Go to the Google Webmaster Blog and check out the recommendations for building smartphone optimized websites. Google clearly prefers responsive web design, which of course, is pretty much common sense since it makes their job easier :) Google mentions the obvious advantages of responsive web design:
Personally, responsive web design should be your default strategy for supporting multiple devices until it is clear that a separate user experience is necessary. Responsive web design is clearly easier to implement, provides a consistent user experience across all your devices, and according to Google will make it easier for SEO.
You may be interested in reading: Mobile Development with Firefox 15 and Responsive Mode and Mobile Web Development for iOS and Android with Adobe Shadow.