Based on some findings by @StroughtonSmith by way of @Xuzz on Twitter who initially discovered higher resolution images, presumably left behind by developers of Apple in version 1.1 and 1.2 of iBooks for iPad, which show that Apple is going to take this pixel-doubled approach again for the iPad 2.
Rafeed.me shows another “x2” iPad graphic found in iBooks 1.2 for iPad, It’s current dimensions are 1536×800 pixels. For comparison, the previous Wood Tile.png in iBooks 1.1 was a mere 768×400 pixels.
Version 1.1 of Apple’s iBooks application seems to have accidentally included some artwork for this hypothetical pixel-doubled iPad. As shown above, the App’s bookmark icon included versions for the iPad, the iPhone and the iPhone Retina Display (iPhonex2). It, however, also included one additional version labeled “iPadx2”. Sure enough, this is exactly double the resolution version of the iPad icon and is distinct from the other versions. The most likely explanation for this added graphic is plans for a double-resolution iPad. Apple has since removed this extra artwork in subsequent versions of iBooks
Based on this information, and persistent rumors of a higher resolution iPad 2, we believe the next iPad will have a 2048×1536 screen resolution. It would also explain why Apple would have to upgrade the GPU on the new devices to drive this higher resolution.
Even at a lower DPI, as the iPad is likely to be held at a distance from your face, less pixels will be required to achieve the “Retina” effect, meaning that the display on the iPad 2 can still be considered a “Retina Display,” and if you remember, Apple also doubled the screen resolution to 960×640 on the iPhone 4 last year.