While responsive web design with CSS media queries is rippling through the web design community, I question its practical use for business websites. Jason Grigsby has an excellent write-up of the technical reasons for skepticism, so I won't go into too much detail reiterating them.
As a solution to the differing needs of mobile browsers, media queries can be used to target CSS styles based on device properties like viewport width. This is a solid technique, but it fails to address what may be greater concerns than design: performance.
Media queries do nothing to satisfy the lower-bandwidth realities of mobile devices - images, scripts, stylesheets, advertising/analytics services (don't get me started on those), and so on are all loaded on the page - even if they end up hidden with CSS. Additionally, scaling images dynamically with CSS can cause major performance hits - particularly if they get hidden.
Anyone who has used AT&T's 3G network in New York City understands why delivering lightweight pages with few HTTP requests to mobile devices is essential. I rarely even browse the web on my iPhone over 3G unless there's a compelling reason - especially when websites don't provide a mobile version of their website. If you're trying to load 500kb or 1mb or more of data over 3G, you should settle in for a wait. Media queries don't help with that.
These sorts of performance issues are more easily circumvented by serving different versions of files for mobile devices. This doesn't necessarily mean a fully separate mobile site, but it does mean doing extra work on the server-side. However, this doesn't mean doubling your work creating a "mobile site" if you've been smart enough to keep your content management and database schemas separate from the application (Rails, Spring, Django, CodeIgniter, etc) which retrieves that content and massages it into an HTML page for delivery.
That said, even with their potentially negative performance issues, media queries are a solid and future-resistant (if not future-proof) method for targeting styles to smaller devices than the desktop. However, they are not a panacea for providing a mobile-targeted experience - in many ways, the more established practice of serving alternate front-end assets is still preferable.