How to use the new iPad (3rd gen) as a second screen?
There is an app called Air Display that will allow you to do this. It also recently got retina display support for the new iPad. 



Some notes:


Use your iPad, iPhone or Mac as a second (or third) monitor with no
messy cables or wires.
Use your finger to draw directly on the screen. Try it with a Mac calculator app, painting tool, or music control surface!
Air Display works in both landscape and portrait configurations. Just rotate your screen, and it automatically reconfigures.


MacRumors and 9to5mac.com discussed the upcoming retnia support update coming soon last week. They issued a press release of some sort last week, many blogs covered this upcoming feature. They also covered this on their blog and how it will work.

Update - it looks like this feature is now available in version 1.6. More on MacStories as well.