A day out in Brighton (didn't get to see the sea though) listening to 12 speakers talk about their experiences with responsive design. The general consensus is that it is hard.

Responsive web design forces us to rethink our (tried and tested) process of designing and building websites. The humble desktop we used to browse the web has now been joined by laptops, mobile phones, tablets, TV's and game consoles. All different shapes and sizes.

Since responsive web design was "born" we had mobile first, content first and now fluid first. If you start from a fluid layout you have a foundation to make progressive enhancements. (Mark Boulton)

Ironically, that's what the web has always been. A plain HTML document without styles is fluid by nature.