Joomla 3 has been touted as being designed for responsive styling. It utilizes Bootstrap which is a front-end responsive styling system set up by two developers from Twitter.
The Protostar template comes packaged with Joomla 3.0 and it has Bootstrap responsive styling built into its stylesheet. The Bootstrap system uses a system of grids that can be set to static or fluid styling, both of which respond well to the various screen sizes that people are using today such as smart phones and tablets.
There is a whole Zemplate website, bj.zemplate.com, that goes into depth explaining the Protostar template, how it works with Bootstrap, module positions, the framework and the @media calls behind it all.