According to Apple, a Retina Display would mean having “pixel density is so high your eye is unable to distinguish individual pixels.” On the iPhone 4S, that translates to a 960×640, 3.5″ display which sports 326 pixels per inch (PPI), which is really dense. When translated over to the iPad 3, you won’t be able to see that level of density on an iPad 3 with a Retina Display, assuming Apple calls it that, and the other reason is, you won’t be holding the iPad 3 so close to your face, either.
At the moment, iPad display manufacturers LG Display and Samsung are able to hit only 2048 x 1536 resolution at 10″, which is double the PPI of the iPad 2 at 264. Perhaps newer developments would see the PPI increase to 326 for the iPad 3 in time?