Retina graphics is a growing phenomenon coming to the web. While Apple is the pioneer of retina resolution devices, they are definitely not going to be the only ones moving into this format. From the iPhone, to the iPad, and even the new Macbook Pro, retina is here to stay and it is something that should definitely be factored into your web design.
Retina graphics are a new technology that double the density of graphics on your website. What HD was to SD television, Retina is to computers. The first time you see it, everything is so crisp and clear. Details are unbelievable. Now, browse to a non-retina web-site and what happens? It's painful to the user. The graphics are blurry, almost like they need a new prescription of eyeware. It gets emphasized, especially when its on the screen next to other retina graphics. If your a company looking to attract these visitors on their mobile devices or retina displays, the last feeling you want to give that user is "Gross".
Below is an example of the jarring effect a retina graphic may have to you when rendered next to a non-retina graphic:
Non Retina Graphic
Marco made a great post covering the new Retina Macbook pro and how it relates to web designers.
I also made a useful Rails gem for retina graphics on the web to make it easy to get started: