I can't say enough good things about responsive website design. Combined with a powerful CMS like Drupal, it becomes an incredibly flexible method of delivering content for any device. It's an efficient way of delivering your content so it looks good and fits in any size display. Content is the key to a website's success or failure and we have become a mobile content society.
It's not exactly new, but it eliminates the need to detect a device type and it's based on the width of a device's viewport. Each degree of transition is known as a breakpoint and based on grids that allow the flexibility by displaying content in that available space without altering the style.
No more mobile versions of select content, now we can display the same content in different ways. One site, many devices—like magic.