Many iPad fans are expecting several hardware upgrades on a next-generation iPad in the spring. Everything from a front facing camera with FaceTime to carbon fiber construction has been discussed, but nobody really knows exactly what Apple has up its sleeve. What's clear is that adding a Retina display to the iPad would be much more difficult than it seems.
Thanks to some calculations by Ryan Block, we can see that a 9.7 inch iPad Retina display would exceed the capabilities of current mobile device technology.
330 pixels per inch (PPI) works on the iPhone 4 precisely because it's such a small screen. Each pixel must be rendered, and the Retina display packs 614,400 pixels at 960 x 640. The same processors are used to display graphics on the iPhone 4 and the current model iPad (the iPad display is 1024 x 768 = 786,432 pixels).
Converting the iPad display to a Retina display would result in a ridiculous number of pixels, leaving graphics processing hardware completely overwhelmed. To match the clarity of the iPhone 4 screen, the iPad would need 2560 x 1920 = 4,915,200 pixels. Not only is this over six times the number of pixels in the current screen, but way more than even these high-quality monitors:
Macbook Pro 17-inch: 1920 x 1200 = 2,304,000 pixels
Dell 24-inch monitor: 1920 x 1200 = 2,304,000 pixels
Apple 27-inch Cinema Display: 2560 x 1440 = 3,686,400 pixels
The fact is that with current technology you just can't render all those pixels on a mobile device. Unless Apple has some top secret graphics processor in the works that will sip power, stay cool and fit inside an iPad, a 9.7 inch Retina display is not coming anytime soon.