iPad 2gen prediction
When Apple updated screen res of the iPhone 4 to 640x960 on the same 3.5" (diagonal) form factor as previous iPhones, the magic Retina Display number turned out to be 326ppi (pixels per inch). The result is an awesome display, the best I have ever seen.
iPad on the other hand has 9.7" (diagonal) with 1024x768 resolution, which gives 132ppi. John Siracusa said that next iPad will most likely have the same improvement in display rez, meaning it will have 2048x1536 - so the iOS4’ @2x API stuff work the same.
Granted, such resolution sounds ginormous - not even Apple’s latest 27" monitor is big enough to design interfaces that big. But if that really happen…
…how big the iPad would physically needs to be?
iPad has 1.33x aspect ratio and 9.7" diagonal display now. 2048x1536 and with 326ppi equals to about 9650in in one very long line, or divide again to get about 29.6in2. From there, the math is easy: 1.33x * x = 29.6, means that x is 4.71in and that physical screen size of the Retina Display iPad would be 4.71 x 6.28, or about 7.85" diagonal.
Sounds quite possible, does it not?