Mac Rumors recently got its hands on a component that's allegedly the display from the forthcoming iPad 3. After putting it under a microscope, the site was able to confirm that the display has a 2048 x 1536 resolution, double the horizontal resolution and quadruple the number of pixels on the iPad 2's current 1024 x 768 LCD.
iFixit has been able to confirm Mac Rumors' findings; Mac Rumors actually mailed iFixit the same display and let them have a look at it. iFixit confirmed that the display has the same overall dimensions as the iPad 2's display (though the "iPad 3" display is 0.6 ounces lighter), and microscopic examination of the display confirmed a 2048 x 1536 resolution at 260 pixels per inch, far higher than the current iPad's 132 ppi.
In its examination iFixit confirmed that this display features a different display connector from the iPad 2 display, so even if you could secure one of these displays on the aftermarket, there doesn't appear to be any means of successfully hooking it up to an iPad 2 or original iPad. No surprises there.
iFixit suggests the new display's ppi might be too low to qualify it as a "Retina Display," while others have quite erroneously suggested that the iPad 2's current display is already Retina quality (in spite of Apple never classifying it that way and despite the current iPad's pixels being easily discernible even with my less than stellar vision). However, we did the math on this last year, when the iPad 2 was rumored to be getting a Retina upgrade, and the numbers tell a different story.
The only consistent definition Apple has ever given about the Retina Display is that individual pixels are indistinguishable from one another. This statement comes with several implied asterisks: pixels are indistinguishable for people with 20/20 vision when held at a reasonable distance. An Air Force bombardier holding an iPhone 4S four inches from his face is going to see those pixels easily, but his 87-year-old grandma isn't going to see them no matter how hard she looks.
Running the numbers shows that this display would meet the "Retina Display" requirements of indistinguishable individual pixels for someone with 20/20 vision if held at a distance from the eye of 13 inches or greater. I don't see too many people holding their iPads closer than that, so despite having a lower ppi density than the iPhone 4S display, the "iPad 3" display easily meets Apple's somewhat fuzzy Retina Display qualifications.
Standard caveats apply: the iPad 3 hasn't even launched yet, so this is all speculative. However, it's worth noting that the iPhone 4's display leaked through the same channel -- aftermarket component resellers -- months before its debut in summer 2010.