Even if the iPad 3 does indeed come with a 9.7-inch, 2048x1536 screen, would that still qualify as a "Retina display"? Does Retina display even have official qualifications? Ever since Apple adopted Retina display as the name for the iPhone 4's high pixel density screen, there's been some confusion as to both of those things. At 330dpi, the iPhone 4 and iPhone 4S seems obvious. At 264dpi, a theoretical iPad 3 not so much.
Heroically, TUAW's Richard Gaywood has busted out the tape measure and done the math, and not just about the iPad 3, and here's his determination.
a pixel-doubled iPad running at 2048x1536 is easily enough definition to count as a Retina display -- even at a 16" viewing distance, which is on the close side from my experimentation with an iPad and a tape measure.
Retina is a function of distance as much as density, so the larger the screen, the further you hold it from your eyes, and the lower the density needs to be to effectively "disappear".
Richard's whole post is outstanding, and includes discussion on Retina display Macs as well, so be sure to hit the link below.