With the release of Magento CE 1.9 and EE 1.14 responsive webdesign for Magneto has been included in the default package.

The responsive template included with Magento lets you easily set up a shop that works just as well on mobiles and tables as on a desktop computer.

From Wikipedia:
Responsive web design (RWD) is a web design approach aimed at crafting sites to provide an optimal viewing experience—easy reading and navigation with a minimum of resizing, panning, and scrolling—across a wide range of devices (from mobile phones to desktop computer monitors).

We have setup a demo for the default RWD template that now comes with Magento which you can view by clicking here…

Resources:
– Magento Responsive Design Demo
– Magento Developers guide to responsive design.
– Responsive Web Design
– Magento Homepage