MacRumors has found artwork in the new Messages beta app with @2X modifier indicating that the new Mac app is ready for ultra high resolution displays on upcoming Macs. According to MacRumors, “The inclusion of the @2x modifier for the high-resolution image is new and lets the system know that the image is the high-resolution variant of the standard image.” MacRumors also noted:

A look at some of the resources from Apple's new Messages app shows several graphics that come in multi-part TIFFs that include regular and double resolution versions. In fact, if you look at the information embedded within the images, you can see the resources were at one point named in the same "@2x" convention. So, it seems Apple's Messages App is already building in support for Retina displays on the Mac.

It has been rumored for quite sometime now that Apple is working to bring ultra high resolution displays with higher DPI to the Mac like they did with the iPhone. And with many Mac applications made by Apple showing @2x modifier and inclusion of HiDPI mode in Lion, it now looks almost certain that we will see a Mac with Retina Display very soon.